Lightly coat a 9x13-inch pan (or 8x8-inch pan if making half the recipe).
In a medium to large bowl, whisk the eggs, milk, and salsa. Set aside.
Heat the oil in a large skillet (preferably non-stick) over medium-high heat. Add the onion and cook until softened (3-5 minutes). Add the ground sausage, red and green bell peppers, garlic powder, chili powder, smoked paprika, cumin, and salt. Stir everything together.
Brown the meat, breaking it into small pieces with a fork (this makes it easier). Use a spatula to mix and blend. Cook until the meat is no longer pink (6-8 minutes). Remove from heat.
Spoon a heaping 1 cup of the meat mixture into the baking dish and spread it out, creating an even layer (it's OK if the bottom has small gaps). Layer with half of the tortillas, overlapping as needed to fit in the pan. Add half of the remaining meat mixture and 1/3 of the cheese, spreading them in even layers. Repeat this process with the remaining tortillas, the rest of the meat mixture, and another 1/3 of the cheese.
Pour the egg mixture evenly over the top.
If refrigerating before baking (recommended but optional): Cover the lasagna tightly with plastic wrap, and refrigerate along with the remaining cheese for at least 2 hours or overnight if preparing ahead.
When ready to bake: Preheat the oven to 350°F (175°C). Remove the plastic wrap from the pan and cover the lasagna with aluminum foil. Bake for 40 minutes. Then, remove the foil, add the remaining cheese (along with enchilada sauce and salsa if using), and bake for another 20 minutes.
To serve: Let the lasagna stand for 5 minutes. Then top with avocado slices, halved cherry tomatoes, Greek yogurt, cilantro, and more salsa if desired. Serve immediately. Enjoy!