Set the oven to preheat at 350°F.
Warm a skillet over medium heat and cook the sausage, crumbling it as it browns. Drain any residual liquid once fully cooked.
Coat the bottom of an oven-safe skillet or baking dish with olive oil, then layer in the squash slices.
Mix the garlic into the browned sausage and distribute this mixture evenly over the squash.
In a separate bowl, whisk together the eggs, whole milk, Italian seasoning, and a pinch of salt and pepper until the mixture is smooth and pale.
Fold in both cheeses and two of the chopped green onion stems, reserving a portion of the cheese if desired for topping.
Evenly pour the egg mixture over the squash and sausage in the skillet, topping with any reserved cheese.
Place the skillet in the oven and bake for 30 to 40 minutes, until the frittata is set and the eggs are fully cooked but not dry.
Sprinkle the reserved green onion over the top as garnish before serving.