Heat up a non-stick skillet over medium-high heat. Spray pan with non-stick spray. Add ground beef and brown all sides, breaking up any clumps; season with salt and pepper. Drain and allow to cool.
Bring vinegar, water, and a pinch of salt to a boil. Pour liquid over cucumbers and set aside to make pickles. Drain the juice when ready to serve.
Whisk together the mustard and lemon juice to make a vinaigrette.
For 1 portion, fill a quart-sized mason jar with 1½ tablespoons of lemon mustard vinaigrette, ¼ cup of tomatoes, 4 ounces cooked ground beef, 2 tablespoons of pickles, 2 cups lettuce, 1 ounce of shredded cheese, and ¼ of the scallions.
Notes
Note: Substitute a low-carbohydrate vegetarian option, such as 1¼ lbs. (total) Impossible Burger, for the ground beef.