Preheat your oven to 350°F and lightly grease a 9x13 inch baking dish or similar size with butter or cooking spray. While the oven heats, prepare all your ingredients: slice the mushrooms into 1/4-inch pieces, remove the tough ends from the asparagus spears and cut them into 1-inch lengths (separating and reserving the tender tips for garnish), and measure out your spinach and shredded cheese. Having everything prepped and ready will make the cooking process smooth and efficient.

In a large skillet over medium-high heat, cook the ground sausage, breaking it into small crumbles as it cooks, until it's browned and cooked through (about 5-7 minutes). Remove the sausage to a plate, then in the same skillet add the asparagus pieces and cook for 1-2 minutes until they begin to soften. Add the sliced mushrooms and cook for another 2-3 minutes until they release their moisture and start to brown. Finally, add the spinach and stir until completely wilted, about 1 minute. I like to cook everything in the same pan to build flavor from the browned bits left behind by each ingredient—it's a subtle touch that adds depth.

Transfer the cooked sausage and all the cooked vegetables from Step 2 to your prepared baking dish, spreading them evenly across the bottom. Sprinkle the freshly shredded cheddar jack cheese evenly over the vegetables and sausage. The freshly shredded cheese will melt more smoothly and create a better texture than pre-shredded varieties.

In a medium bowl, whisk together the eggs, milk, garlic powder, and a pinch each of salt and pepper until well combined and slightly frothy. Pour this mixture evenly over the cheese and vegetables in the baking dish, making sure the liquid flows into all the corners and gaps. This custard-like mixture will set up during baking and bind all the flavors together into a cohesive dish.

Place the baking dish in the preheated 350°F oven and bake for 15 minutes. Carefully remove the dish from the oven and arrange the reserved asparagus tips on top in a decorative pattern, then return to the oven and bake for another 5-10 minutes until the egg mixture is set but still slightly jiggly in the very center (it will continue to cook slightly as it cools). The frittata is done when a knife inserted near the center comes out mostly clean. Let it rest for 5 minutes before serving—this helps the eggs set completely and makes it easier to slice.