Preheat oven to 400°F.
In a large bowl, gently combine ground elk, cheddar cubes, panko breadcrumbs, egg, Worcestershire sauce, mustard, and salt. Mix just until combined. Do not overmix.
Form the mixture into a loaf shape and place in a lightly greased baking skillet or on a lined sheet pan.
Bake for 20 minutes.
While the meatloaf bakes, prepare the gravy. Heat butter and olive oil in a skillet over medium heat. Add diced onion and cook until softened.
Stir in sliced mushrooms and rosemary. Cook until mushrooms release their moisture and begin to brown.
Sprinkle flour over the mixture and stir to coat. Cook for 1 minute.
Slowly whisk in beef stock. Add Worcestershire sauce and salt. Simmer until thickened.
Remove meatloaf from oven after the first 20 minutes and spoon the mushroom gravy over the top.
Return to oven and bake an additional 20–25 minutes, or until internal temperature reaches 160°F.
Let rest 5–10 minutes before slicing and serving.