Preheat your oven to 375°F. While it heats, prepare all your ingredients: finely dice the onion into 1/4-inch pieces, thinly slice the mushrooms, mince the garlic, cube or chop the ham into bite-sized pieces, drain the jarred red peppers and chop them if needed, and measure out the spinach. Having everything prepped and ready will make the cooking process smooth and prevent any rushed moments.

Melt the butter in a large 12-inch oven-safe skillet (or similar size) over medium-high heat. Add the diced onion and sliced mushrooms, cooking for about 5 minutes, stirring occasionally, until the mushrooms begin to release their moisture and soften. Add the minced garlic and cook for another 30 seconds until fragrant. The mushrooms should be tender and lightly golden—this develops their flavor significantly compared to adding them raw.

Stir in the chopped ham, red peppers, and spinach to the skillet. Cook for about 3 minutes, stirring frequently, until the spinach is completely wilted and any excess moisture from the vegetables has reduced slightly. This helps prevent a watery frittata and ensures all components are warm and evenly distributed throughout the pan.

While the vegetables cook, whisk together the eggs, grated cheese, and sour cream in a large bowl until well combined and smooth—the sour cream should be fully incorporated to ensure an even, creamy texture throughout. Once the vegetables in the skillet are ready, remove it from heat and pour the egg mixture directly over them, then gently stir to distribute all ingredients evenly. I like to give it a good 30 seconds of stirring to ensure the ham and peppers are well dispersed throughout.

Transfer the skillet to the preheated 375°F oven and bake uncovered for approximately 30 minutes, until the eggs are set but still slightly jiggly in the very center—this is when the frittata will have the best texture as it continues to cook slightly from carryover heat. The top should be lightly golden and the edges should pull slightly away from the sides of the pan.

Remove the frittata from the oven and let it sit in the skillet for 5 minutes before slicing. This resting period allows the eggs to fully set and makes it much easier to cut clean slices without the frittata breaking apart. You can serve it warm directly from the skillet or slide it onto a cutting board for more attractive presentation.