Preheat oven to 400F. Lightly grease a 9×13 baking dish and set aside. On a large baking sheet, combine sweet potatoes with 2 tbsp olive oil, 1/2 tsp kosher salt, and 1/4 tsp freshly ground black pepper; toss to coat well. Bake in a single, even layer for 20 minutes until soft then remove sweet potatoes from oven.
Meanwhile, heat remaining 1 tbsp olive oil on a large skillet over medium high heat. Add onions and garlic and stir 2-3 minutes or until translucent. Add peppers and stir 1 more minute, then add sausage to the same skillet with the peppers and onions. Cook and break apart sausage with spatula until cooked through. Drain and add to baking dish. Add roasted sweet potatoes and chopped spinach to the mixture in baking dish. Gently toss to combine all ingredients well.
In a bowl, gently whisk together the eggs, milk, 1 cup of cheese, 1 tsp kosher salt, and 1/4 tsp freshly ground black pepper until well combined. Pour evenly over the sausage/potato mixture in baking dish. Top with remaining ¼ cup cheese and bake uncovered about 25-30 minutes or just until eggs are set in center.
