In addition to what has already been said, I broke down the Mongol warriors diet here. A copy/paste from that answer follows:

Mongol Warrior's Diet

Dr. Timothy May cites a study which said that 3,600 calories of food were necessary for active Mongol males in a day. This figure is what is estimated to be necessary to ward off starvation and malnutrition. Each man carried his own rations and his arban consisting of ten men carried three and a half dried sheep and a cauldron with which to cook the meat. Standard rations were one skin of water and a paste made out of powdered milk.

Sources of Food

The preferred drink of the Mongols was something called kumiss. It is fermented mare's milk. William of Rubruck, a Flemish Franciscan missionary, observed the making of kumiss.

He [Rubruck] described how milk was poured into a large leather sack and then churned with a specially crafted stick or club, the head of which was hollowed out. Part of the mixture turned to butter, and the milk fermented and soured. The butter was then removed. More churning increased the fermentation and produced a clear and more potent liquor.

Dairy products, such as kumis were a large part of the Mongol's diet. Timothy May estimates that a mare produces roughly 2.25-2.5 quarts of milk per day above what is necessary to sustain offspring. Mare's milk is roughly 20kcal/oz which means that 2.25-2.5 quarts of milk can sustain someone for 1,400-1,600 kcal/day. That's about a third of the 3,600 calorie diet of a Mongol soldier. In addition, they prepared a paste made of milk called qurut. This paste is made from powdered milk. When necessary, usually during winter when milk was not as abundant as usual, the paste would be put into water, blended, and drunk. An estimated 10 pounds of dried milk is taken on campaign. The same study cited earlier says that this form of ration provided at least 800kcal and 80 grams of protein. Again, another large chunk of the 3,600 calorie diet.

If dairy meals such as the ones described above were not available, the Mongols would boil millet in water to make a thin soup-like mixture. This provided nutrition. Usually they would eat some meat, probably sheep, with this soup. William of Rubruck also described what happened when a horse or ox died on campaign.

If a horse or oxen died they promptly dried the meat or made sausages;some they ate there and then, and the rest were smoked for later use.

Meat was prepared by boiling in a cauldron with salt and some water. This stew was called shülen. When meat was available either from a dried sheep or from something obtained while hunting, this was the main means of cooking it. The Mongols didn't have many other ways of preparing meat other than boiling while on campaign. Another interesting fact is that the Mongols also used the blood of their horses to sustain themselves. This sounds crazy and I think it just might be. Marco Polo observed the following:

When they are going on distant expedition they take no gear with them except two leather bottles for milk; a little earthenware pot to cook their meat in, a little tent to shelter them from rain. And in case of great urgency they will ride ten days on end without lighting a fire or taking a meal. On such occasion they will sustain themselves on the blood of their horses, opening a vein and letting the blood jet into their mouths, drinking till they have had enough, and then staunching it.

It sounds crazy, right? Well, Timothy May did the research on how many calories come from a pint of horses blood! He estimates that a horse can donate one third of its blood without any serious health risks. You can reason that the horse will be fatigued, but major health effects would not be a factor here. That means that a horse can provide about 14 pints of blood, each pint can supply 156kcal per pint. So, approximately 2,184 kcal or almost 2/3 of the 3,600 calorie diet of a Mongol. Each soldier took between 5-8 horses with them on campaign. The maximum eight horses could supply a warrior with approximately 6 days of full rations. Timothy May does note that this would take a toll on the horses and it was probably used as a last resort, not as the first choice when looking for food.

When cereals were available, they were used to thicken soups or as a porridge.

Sources and Additional Reading

The Mongol Art of War by Timothy May

The Empire of the Steppes: A History of Central Asia by Rene Grousset

The Mongols and Global History by Morris Rossabi

The History of Mongol Conquests by J. J. Saunders

EDIT: Formatting