Heat your oven to 375°F and grab a 10-inch oven-safe skillet. Pour in 2 tbsp olive oil and let it get shimmering over medium-high heat. This takes about 1 minute, but don't skip this step because hot oil makes your vegetables cook faster.

Toss in the diced onion and cook for about 2 minutes until it starts getting soft. Add the diced zucchini and red bell pepper next, stirring occasionally for 3-4 minutes until they soften slightly. You're looking for them to be tender but not completely cooked—they'll finish in the oven.

Stir in the minced garlic and fresh spinach, cooking for just 1 minute until the spinach wilts down. The kitchen smells amazing right now, which is honestly my favorite cooking moment. Once everything's combined, spread it evenly across the bottom of the skillet.

Whisk together 8 eggs, 1/4 cup milk, salt, black pepper, and dried oregano in a bowl until everything's combined. Pour this mixture right over your vegetables in the skillet, making sure it flows down to the bottom. Scatter the cheddar cheese over the top evenly.

Cook on the stovetop for 2 minutes until the bottom starts setting slightly. You'll see the edges firm up a little, which means it's time to transfer to the oven. Don't crank the heat too high on the stovetop or the bottom burns before the eggs set.

Slide the skillet into your 375°F oven and bake for 12-15 minutes until the center is just set. The top should be golden and puffy when it comes out. My oven tends to cook a little faster, so I check at minute 12 just to be safe.

Let the summer vegetable frittata family rest for 2 minutes before slicing into 6 wedges. This cooling time helps everything hold together better, and it's still piping hot when you plate it up. A little rest makes slicing way cleaner than diving right in.