Preheat oven to 375°F and position the rack in the middle.
Heat 1 Tbsp olive oil in a large oven-safe skillet over medium heat. Add 2 cloves minced garlic and 1 small chopped onion; sauté about 4 minutes, until the onion is tender and the garlic is fragrant.
Add 1/2 red bell pepper (chopped), 1/2 green bell pepper (chopped), 1/2 zucchini (shredded), 1/2 cup broccoli florets, and 1/4 cup sun-dried tomatoes (chopped). Cook, stirring occasionally, about 2 minutes until the vegetables are warmed and beginning to soften. Remove the skillet from heat and spread the vegetables evenly in the pan.
In a large bowl, whisk together 6 eggs, 2 Tbsp almond milk, 1/4 cup Parmesan cheese, 1 tsp dried oregano, 1/4 tsp red pepper flakes, 1/2 tsp pepper, and a pinch of sea salt until well combined and slightly foamy.
Pour the egg mixture evenly over the vegetables in the skillet. Return the skillet to medium heat and cook until the bottom begins to set and the edges start to firm, about 5 to 6 minutes.
Transfer the skillet to the preheated oven (or transfer the contents to an ovenproof baking dish if your skillet is not oven-safe) and bake 10 to 15 minutes, until the edges are slightly brown and a knife inserted in the center comes out clean.
Remove from the oven, let the frittata rest 2–3 minutes, then slice and serve.