Heat oil in a heavy, nonstick, oven-safe skillet. (If you don’t have an ovenproof skillet, no worries. There is a tip at the end.) Add wet spinach and cover and steam for 2–3 minutes. Lift cover and stir spinach with a wooden spoon. Cover and steam an additional 2 minutes until all the spinach has wilted. Don’t overcook.

Remove spinach from skillet and drain in a colander, making certain to squeeze out the water with a wooden spoon. Set aside.

Add additional oil to skillet and heat until it shimmers. Add onion and sausage to skillet. Continue to stir and break up the sausage until it is cooked through and has crumbled. Drain any excess fat. Remove from heat.

Separate spinach with two forks. Sprinkle the separated spinach over the sausage and onion mixture. The spinach will try to clump up, so keep separating it until it is mixed together with the sausage.

Sprinkle the top of the sausage and spinach evenly with cheeses.

Beat eggs in a large bowl for a couple of minutes. Add remaining ingredients and whisk until blended. Pour mixture into skillet. Let mixture cool, then cover with aluminum foil or lid and transfer skillet to the refrigerator (or transfer into a greased, 9" x 13" casserole dish).

When you are ready to cook the frittata, take the skillet out of the fridge, uncover and preheat oven to 350 degrees.

Place skillet in oven and bake for 35–40 minutes or until the middle is set and bounces back to touch or a knife inserted into the middle comes out clean.

If this recipe goes from stovetop directly to your oven, bake for only 30–35 minutes.