For the pasta: In a large pan with sides and a lid, add the olive oil over medium heat. Add the sage and fry until just crisp, 2 to 3 minutes. Remove to a paper towel-lined plate and season with a tiny pinch of salt.
Add the mushrooms to the pan and cook, stirring, until wilted and darkened, 5 to 8 minutes; season with salt and pepper. Add the pasta, peas and chicken stock. Cook, stirring and tossing, until all the liquid is absorbed, 1 to 2 minutes.
For the 1-2-3 alfredo sauce: Add the garlic to the pan and toss, then add the cheese and heavy cream. Stir to coat everything, then cover and reduce the heat to low. Cook, covered, until the sauce has combined and thickened slightly, about 8 minutes.
Top each portion with a fried sage leaf.