This Philly Cheesesteak Spaghetti Squash Casserole is a low-carb twist on the classic Philly cheesesteak sandwich. Tender spaghetti squash is mixed with sautéed onions, bell peppers, and thinly sliced steak, then topped with melted provolone cheese. It's a hearty and flavorful dish that's perfect for a weeknight dinner or a crowd-pleasing potluck.