Step 1 Prep oven and baking pan. Heat oven to 400°F. Line a large baking sheet (or two medium baking sheets) with parchment paper; set aside. Lightly mist a 9 x 13-inch baking pan with cooking spray; set aside.
Step 2 Roast the vegetables. Spread the mushrooms, potatoes, red bell pepper, and onion out on a large baking sheet (or two medium baking sheets). Drizzle evenly with the oil, and season with a few generous pinches of salt and pepper. Toss the veggies until they are evenly coated with the oil and arranged in an even layer, not overlapping. Bake for about 20-25 minutes, or until the potatoes are tender. Remove baking sheet(s) from the oven, and carefully transfer the veggies into the prepared baking dish; set aside.
Step 3 Brown the sausage. Meanwhile, as the veggies are roasting, cook the sausage in a large sauté pan over medium-high heat until browned, using a wooden spoon to break up and crumble the sausage as it cooks. Once the sausage is browned, add in the garlic and kale and sauté for an extra 2-3 minutes, stirring occasionally, until fragrant. (If there is not leftover oil in the pan from the sausage, you may need to add in an extra teaspoon or two to sauté the garlic and kale.) Remove pan from heat, and transfer the sausage into the prepared baking dish; set aside.
Step 4 Whisk the eggs. In a separate large bowl or measuring cup, whisk together the eggs, milk, Old Bay seasoning, plus an extra 1 teaspoon salt and 1/2 teaspoon black pepper until combined.
Step 5 Assemble the casserole. Once the veggies and sausage have been cooked and added to the baking dish, use a spoon to give them a brief toss. Then spread the mixture out in an even layer in the baking dish. Carefully pour the whisked egg mixture evenly on top of the veggies and sausage.
Step 6 Bake. Bake uncovered for 30-35 minutes, or until a toothpick inserted in the middle of the casserole comes out completely clean. (If the veggies on top of the casserole get too browned before the casserole is done, just lay a piece of aluminum foil gently on top of the baking dish until the inside is cooked through.) Remove from the oven and transfer to a cooling rack to cool for 10 minutes.
Step 7 Serve warm. Then slice, serve, and enjoy! Or cover and refrigerate for up to 3 days.
