Preheat your oven to 375°F and whisk together eggs, milk, salt, pepper, and oregano in a large bowl. Beat until the mixture is light and slightly foamy, about one minute.
Heat olive oil in a 10-inch oven-safe skillet over medium-high heat. Add diced red onion and minced garlic, sautéing for two minutes until fragrant and softened.
Add zucchini and red bell pepper to the skillet, cooking for three minutes while stirring occasionally. The vegetables should begin releasing their moisture but remain slightly firm.
Stir in cherry tomatoes and fresh spinach, cooking for exactly one minute until the spinach just wilts. Pour the egg mixture over the vegetables, spreading everything evenly across the skillet.
Sprinkle cheddar cheese and fresh basil over the cozy summer veggie frittata recipe, then immediately transfer the skillet to the preheated oven. Bake for 12-15 minutes until the center is set but still slightly jiggly—it should wobble just a bit when you shake the pan.
Remove from the oven and let the frittata rest for two minutes before sliding onto a cutting board. Cut into four wedges and serve warm or at room temperature.