Bring a large pot of lightly salted water to a boil. Cook lasagna noodles in the boiling water, stirring occasionally until tender yet firm to the bite, about 8 minutes. Drain.
Heat olive oil in a skillet over medium heat. Add fennel, onion, mushrooms, and garlic; cook and stir until onions are translucent, 5 to 7 minutes.
Place sausage into a skillet over medium-high heat. Cook and stir until browned and crumbly, 5 to 7 minutes; drain and discard grease. Add marinara sauce, basil, and red pepper flakes to skillet, stir to combine.
Coat the bottom of a Dutch oven with marinara sauce mixture; add a layer of lasagna noodles. Spread a 1/4-inch layer of cream cheese over noodles; add a layer of onion-fennel mixture. Top with more sauce. Repeat layers until all sauce, cream cheese, and onion-fennel mixture are used up; about 3 layers. Top last layer with mozzarella cheese.
Bake in the preheated oven until bubbling, about 30 minutes.