Prep the ingredients. Dice 4 slices thick-cut bacon if using. Peel and thinly slice 2 small Yukon gold potatoes. Mince 2 garlic cloves. Pick the leaves from 1 small bunch fresh thyme until you have 2 teaspoons. Grate 1 cup Gruyère, Fontina, or cheddar cheese.

Heat the oven. Arrange a rack in the middle of the oven and heat the oven to 400°F.

Whisk the eggs 6 to 8 large eggs (use 6 if using bacon and cheese) and cream together. Add, ½ teaspoon of the kosher salt in a medium or large bowl and whisk to combine.

Cook the bacon in a cold 10- to 12-inch nonstick ovensafe frying pan or cast iron skillet over a medium-high heat, stirring occasionally, until browned and crisp. Remove the bacon to a paper towel-lined plate. Pour off all but 2 tablespoons of the fat. (If not using bacon, heat 2 tablespoons oil in the skillet, then proceed with adding the potatoes).

Saute the potatoes in bacon fat. Return the pan to medium heat and season with ¼ teaspoon black pepper and the remaining ½ teaspoon kosher salt. Cook, stirring occasionally, until tender and lightly browned, 4 to 6 minutes.

Wilt the spinach with the garlic and thyme. Add the garlic, thyme, and 2 cups baby spinach. Cook, stirring often, until the spinach wilts, 30 to 60 seconds. Return the bacon to the pan and stir to evenly distribute.

Add the cheese. Spread the vegetables into an even layer. Sprinkle the cheese evenly over the top. Let cook until just starting to melt.

Pour the egg mixture into the skillet. Pour the egg mixture over the vegetables and cheese. Tilt the pan to make sure the eggs settle evenly over all the vegetables. Cook until the eggs at the edges of the pan are just beginning to set, 1 to 2 minutes.

Bake the frittata for 8 to 10 minutes. Transfer the pan to the oven. Bake 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 a few minutes more. For a browned, crispy top, turn on the broiler for a minute or two at the end of cooking.

Cool and serve. Let the frittata cool in the pan for 5 minutes. Slice into wedges and serve.