Bring a salted pot of water to a boil. Preheat oven to 450
In a pan over medium heat add a splash of olive oil and add in the sausage and onion. Cook, stirring often, for about 5 minutes while breaking up the sausage.
Turn the heat to high and add in the mushrooms. Cook on high heat about 10-15 minutes until everything is browned and much reduced in size.
Turn the heat on the mushrooms back to medium and add the butter to melt. Add in the flour and cook, stirring, for about 5 minutes to lightly toast the flour and make sure there are no clumps.
Meanwhile, add the pasta to the boiling water and cook to the package instructions for al dente.
Stir in the milk to the mushroom pan. Bring to a simmer. The sauce will thicken. Remove from heat.
Strain the pasta and add it right into the mushroom mixture. Stir. Add in about 1/4 of the cheese and stir until it is all fully melted. Repeat with the next 1/4, then the third 1/4. It probably won't all melt in the third time.
Pour half of the pasta mixture into a baking dish. add half the remaining cheese and half the sriracha. Stir slightly. Repeat to fill the dish.
Mix the melted butter with the breadcrumbs and add them to the top of the baking dish.
Bake at 450 for about 20 minutes until bubbling and browned on top.
Wait 10 minutes before serving