Remove beef from refrigerator and let stand at room temperature for 1 hour; pat dry.
Preheat oven to 425 degrees F.
Season beef with salt and pepper.
Stir together 3 tablespoons butter and both mustards. Spread mixture over tenderloin to cover.
Place beef on a rimmed baking sheet. Arrange squash around beef and brush squash with remaining butter; season with salt and pepper.
Roast until an instant read thermometer inserted into center of the beef reaches 130 degrees F (medium rare), about 30-40 minutes depending on thickness of the cut. Flip squash half way through cooking.
When cooked through, remove baking sheet and let beef rest 15 minutes before slicing.
Toss frisee, parsley, and squash with vinegar and oil and season with salt and pepper.