We loosely followed two recipes: https://cafedelites.com/breakfast-egg-muffins-3-ways/ and https://www.williams-sonoma.com/m/recipe/frittata-sausage-wild-mushrooms-cheddar.html. Here is the recipe: first, preheat oven to 350 and prep the ingredients: chop 2-3 shallots (we used an electric handheld chopper). Roughly chop 6 veggie sausage patties so that they are crumbled (we used a 8 oz package of 6 Morningstar original sausage patties, which we also chopped in the handheld chopper). Wash, dry, and slice 3.5 oz (about 6-7 medium) fresh shiitake mushroom into ½ inch x ¾ inch slices. Prepare about ½ cup spinach; we used frozen chopped spinach, so we rinsed and then towel-dried it to squeeze out all excess water; this is important to avoid watering down the frittata. Shred about 1 cup of sharp cheddar cheese, and divide in half (two ½ cup portions). Second, lightly spray your muffin tins with oil; we ended up filling 20 muffins, so we used two 12-muffin tin pans. Third, saute the chopped shallots in a little oil (we used a 10-inch cast iron pan) on medium heat, lightly seasoning with black pepper. When wilted and fragrant, add the shiitakes. Saute for 1 minute, then add the crumbled veggie sausage. Add a little oil if needed. Saute until sausage is slightly browned and fragrant, then remove from heat and let cool slightly. Fourth, crack a dozen large eggs in a large mixing bowl. Add a splash (2 Tbsp) of milk. Whisk, then stir in the sausage mixture. Add in the spinach, making sure to the spinach is well dispersed in the egg-sausage mixture (because we used frozen, squeezed spinach, we had to make extra effort to separate the pieces out to avoid large chunks of spinach). Fold in half of the shredded cheese. Fifth, using a small ladle, evenly distribute the mixture into the muffin tins, so that each filled tin is about 2/3 full (in our case, we filled 20 of 24 available muffin tins). When finished, evenly distribute the remaining ½ cup of shredded cheese (about ½ tsp per muffin). Finally, bake at 350 until done (about 20-25 mins). We plan to enjoy these for Sunday breakfast and have the remainder for breakfast at work; the Cafe Delite link provides tips on storage. (Aug. 2018)
Note: We found the dish very egg-yolk heavy and dense. The next time, we may try (1) a mixture of egg whites and whole eggs or (2) try a 3 egg to 1 cup milk ratio such as here: http://piglettedc.tumblr.com/post/114716332676/sausage-leek-and-tomato-frittata-saute-1-hot