Step 1

Preheat oven to 375°F. Coat a muffin tin with nonstick cooking spray.

Step 2

In a large skillet over medium-high heat, heat oil until shimmering. Add mushrooms and 1/4 teaspoon salt and cook until softened and the mushrooms have released most of their liquid, about 5 minutes.

Step 3

Using a slotted spoon, remove mushrooms from skillet to a large mixing bowl, leaving any remaining oil and liquid in the skillet.

Step 4

Add to the skillet with water and sauté until tender and wilted, about 10 minutes. Drain and place in a clean kitchen towel. Squeeze and twist the towel to remove as much liquid as possible.

Step 5

Sauté in pan until heated through, about 5 minutes. Drain and place in a clean kitchen towel. Squeeze and twist the towel to remove as much liquid as possible.

Step 6

Add the spinach to the bowl with the mushrooms. Stir in eggs, cheese, 1/2 teaspoon salt, and 1/4 teaspoon pepper.

Step 7

Divide egg mixture evenly among 12 muffin cups. Bake until a toothpick inserted comes out clean, about 25 minutes.

Step 8

Cool slightly before removing from muffin tins (the egg cups should come out easily). Serve hot or at room temperature. Store leftovers in the refrigerator and use within 4 days.

Step 9

Cool and transfer to a single layer on a plate. Freeze until solid, at least 30 minutes, then transfer to a freezer-safe bag. Freeze up to 2 months.

Step 10

Microwave them straight from the freezer until heated through, about 1 minute.