Cook the elbow macaroni according to package directions until just tender. Drain and set aside, reserving a splash of pasta water for later if needed.

In a large skillet over medium-high heat, brown the ground beef, breaking it into bite-sized pieces. Once the meat loses its pink color and develops golden edges, drain excess fat and season with salt and pepper.

Reduce heat to medium and add the onion soup mix to the browned meat, stirring for 30 seconds until fragrant. Pour in the cream of mushroom soup, milk, and butter, whisking until smooth.

Fold in the cooked pasta, frozen vegetables, and half the cheese. If mixture seems too thick, add a splash of reserved pasta water. Transfer to a greased 9x13 baking dish and top with remaining cheese.

Bake at 350°F for 20-25 minutes until the cheese melts and edges bubble gently. Let rest for 5 minutes before serving.