I'm trying to understand what such a diet would look like for a person living in Fereldan which is basically fantasy England. I would like some help on this because the Templar's seem to require a similar of not greater amount of food then normal knights.
I'm not sure you really mean 10,000 kcal, unless the knights of Fereldan are also giants. We know from account books that galley crewmen in the Mediterranean drank about 4 liters of water a day and consumed 4000-5000 kcal, and that was sufficient for constant strenuous labor!
And actually, naval menus aren't a bad place to start if you're looking for a kind of no-nonsense, "eat to live" military diet. Helpfully, they were pretty standard around the Mediterranean and still used by the Spanish navy into the 1700s, which speaks volumes to the effectiveness if not the palate!
In addition to the water, wine or beer provided more liquid with calories. The main meal consisted of bread, often a hard biscuit; and a sort of very thick stew. The stew typically had salted meat, beans, onions, garlic, maybe some spices if any were on hand, and a fat (olive oil being ideal for preservation reasons). Generally, the meal was consumed by mashing up the biscuit into the gruel to form, I guess, a consistency somewhere between Silly Putty and goop. Crew members also usually received a ration of cheese. Muslim crewers would use that instead of pork in their stew, but it's unclear to me whether Christians consumed it separately or as another mix-in.
Now, that's for a sea-based diet, which is of course going to be much more limited. I've discussed the types of foods common in late medieval English diets in this earlier answer, which I think gives a good basis for filling in that 4000-5000 kcal marker observed in naval diets. A lot of carbs (bread), a lot of protein (many many varieties of meat). For wealthy knights, it would have been ideal to use only the most expensive white flour for their bread; brown flour, bean flour, and worst of all pea flour were for Those Peasants. Meat was the other great class marker in terms of diet. Peasants were lucky to get any; nobles consumed pounds and pounds of it. For purposes of preservation, a lot of it would have been salted. Salt fish was the great urban winter staple of the late Middle Ages, as well as a ubiquitous food in monasteries (since you mentioned the Templars) and during Lent.
As far as the occasional feast goes, I've discussed a couple of wedding feast menus before that might interest you.
While you wait for an answer, there are many previous discussions about medieval food. Here are a few. Though these don't directly mention calories they give an idea about what people might eat: