Heat the oven. Arrange a rack in the middle of the oven and heat to 350°F.

Whisk the eggs, milk, salt and pepper in a small bowl. Set aside.

Cook sausage in pan, breaking up, stirring occasionally, 8 to 10 minutes. Remove to a paper towel-lined plate and pour off all but 2 teaspoons of the fat.

Return the pan to medium-heat, add the potatoes, stirring occasionally, for 4 to 6 minutes or until tender and lightly browned. Add peppers near end.

Pile the spinach into the pan with the garlic and thyme, and cook, stirring, for 30 seconds to 1 minute or until spinach wilts. Add sausage back to the pan and stir to evenly distribute.

Spread the vegetables into an even layer, flattening with a spatula. Sprinkle the cheese on top and let it just start to melt. If making mini frittatas in muffin tin, toss everything and divide among cups.

Pour the egg mixture over the vegetables and cheese. Tilt the pan to make sure the eggs settle evenly over all the vegetables. Cook for a minute or two until you see the eggs at the edges of the pan beginning to set.

Bake the frittata in the oven until the eggs are set, 8 to 10 minutes. To check, cut a small slit in the center of the frittata. If raw eggs run into the cut, bake for another few minutes; if the eggs are set, pull the frittata from the oven. For a browned, crispy top, run the frittata under the broiler for a minute or two at the end of cooking.

Cool in the pan for 5 minutes, then slice into wedges and serve.

Note: I did a test run with no potatoes, 5 eggs, 1/3 cup milk, kale instead of spinach, 1/4 cup cheese (for half of the muffins). It made 8 muffins