Heat oven to 350 degrees Fahrenheit.
Mince 1 small onion (any color) (can sub 2 shallots) and 1 green bell pepper (can sub red, orange, or yellow bell pepper).
Wipe any dirt from 1 8 oz. package of cremini or white button mushrooms (or mushrooms of choice) and chop into small pieces (feel free to either use or discard the stems).
Dice 2 cup of cooked ham into cubes. Make sure the ham and veggies are cut relatively the same size so that everything cooks evenly.
Grate 1 cup of sharp cheddar cheese (or cheese of choice).
Place a large skillet on the stove top and heat to medium-high. Add 2 Tbsp. of olive oil to the skillet. Once heated, add the diced ham to the skillet, leaving the rendered fat and oil. Sauté for 5-6 minutes, stirring often. Remove the sauteed ham from the skillet and set aside to cool.
Add the minced onions, bell peppers, and mushrooms to the skillet along with ¼ tsp. each kosher salt and pepper. Sauté the veggies for 8-10 minutes, stirring often. Remove the skillet from the stove top and allow the cooked veggies to cool. Crack 10 eggs into a bowl and add ¼ tsp. each kosher salt and pepper. Whisk the eggs until completely combined. Add the cooled ham and sauteed veggies and grated cheese to the egg mixture and stir until combined.
Generously spray 2 muffin tins with cooking spray to prevent the egg cups from sticking to the surface as they bake. Pour an equal amount of the egg mixture in each muffin cavity, being careful not to overfill them (this recipe yields 16-18 egg muffins, so there will be some empty muffin cavities).
Place the muffin tins in the oven and bake for 24-25 minutes, or until the egg muffins are set.
Remove the muffin tins when finished and allow the egg cups to cool slightly. Once the muffin tins have cooled, run a butter knife along the edges and carefully remove the egg cups.
To Serve: Enjoy right away or allow to cool, place in an airtight container, and store in the refrigerator for 3-4 days.