Sauté the aromatics by heating the olive oil in a large soup pot over medium heat until it shimmers. Add the chopped onion along with a sprinkle of kosher salt and black pepper, cooking for about 5 minutes until fragrant and beginning to caramelize.
Incorporate the chopped red bell pepper, minced garlic, ground spicy Italian sausage, and lean ground beef into the pot. Cook everything together until browned all over, breaking up the meat as it cooks—this should take around 10 minutes.
Add in the chopped tomatoes from the can along with tomato paste, dried oregano, bay leaf, whole milk, and about 1 cup of water; season with additional salt and pepper to taste. Stir thoroughly to combine these ingredients then let them simmer over medium heat for approximately 30 minutes.
Cover your pot and reduce heat to low allowing it to simmer gently for up to 3 hours; stirring occasionally enhances flavor significantly as it develops over time.
In a separate medium bowl, combine your ricotta cheese with basil pesto until well mixed—this will be a key component in layering your lasagna later on.
Preheat your oven to 375 degrees F while you grease a 9x13 inch baking pan lightly with olive oil or cooking spray; then layer everything starting from spreading a quarter of your prepared Bolognese sauce at the bottom followed by lasagna sheets topped off by half of that creamy ricotta mixture! Continue layering until finished then bake uncovered for about 45 minutes or until golden brown on top before serving!