There are a few things to consider before answering the question. First of all, what constituted lunch, and at what time it was taken, varies greatly with geographic area and with historic period. Some times there might have been a hot lunch (and cold dinner), an earlier lunch, a luncheon, a supper. The concept of lunch started to take hold in the Industrial Revolution, but it was not called lunch yet.

Second, the question seems to presume that a field worker would assemble a sandwich, which was probably not the case. The actual sandwich (two slices of bread with something in between) was invented around 1750 but it will have taken some time to become popular even among field workers. They might have eaten open faced sandwiches or they would eat a chunk of bread and/or a bite of cheese, as we will see in a minute.

From various literature on farming we know that it was very common for field workers to return to the home for a lunch (could be bread and cheese but it could have been a hot lunch) or for lunch to be brought out into the field. Evidence of this can be seen in Wilder's Farmer Boy, where the family is called back to the house for a midday meal, or When I was a girl in Holland where the meal is brought out to the farmers. The author describes one of the younger farmer daughters bringing a kettle of tea to the workers in the fields, along with rye bread, butter and cheese. During harvest meals were simple and sparse, just enough to refuel and get back to work.

Field were attached to farms, and most field hands stayed with a family for a season (starting in May generally). Therefore, they would receive meals from the farmer's family. Day hands might receive meals or might return to their home (in the case of married men) to enjoy the meal there.

To give you some visual on meals taken in the field, I found this artwork by Pieter Brueghel The Elder, The Harvesters:

https://www.metmuseum.org/art/collection/search/435809

Here we see that they indeed took the shape of picnics, with a big basket with supplies. Here you see loaves of bread sliced on the field, spread with butter (there is a tray with butter and the people eating bread also have butter.) There is porridge in bowls, there is a drink in a big jug, there seems to be a large chunk of cheese to slice from. There are also some pears on the central napkin. Even though this artwork is not your exact location or time period, it gives a very nice general image of a break and meal in the field.

Another image of how food items were transported can be seen here, The Baker's Cart by Jean Michelin:

https://www.metmuseum.org/art/collection/search/437082

Clearly a baker would have access to bread, but it's interesting to note that there are several small bottles (one with an insulation layer of straw or reed around it, with would make the bottle more durable too), and a little glass. The description tells us this is medicinal eau de vie, but it gives a nice image of how fluids would be transported. The napkin is a key element in lunches during work time, to wrap items in and to spread on a surface and keep your meal clean.

In cities and larger villages, food stalls and larger establishments were also common to serve food. Food stalls started in the Middle Ages and sold portable types of food, think meat pies, pastries, or pancakes. (From Lunch, a history) Starting from the Industrial Revolution, canteens and cafeteria would serve a quick lunch for factory and other workers.