Place potatoes and smashed garlic in a medium saucepan, cover with cold water. Add salt totaste. Simmer until potatoes are tender, about 15 minutes. Drain.
Add 1/4 cup butter, cream cheese, and dried mustard to hot potatoes. Mash until smooth.
Cook sausage in a skillet over medium heat until well browned and cooked through. Remove sausage from skillet and place in the oven to keep warm. Reserve 1 Tbsp drippings.
Melt 1 Tbsp butter in reserved sausage drippings. Add onion, sauté over medium heat until well browned and soft, about 20 minutes. Add chopped garlic, cook until fragrant.
Add beef broth and red wine to onions, simmer until liquid is reduced by half, about 30 minutes.
Season with salt and pepper to taste. If a thicker sauce is desired, mix cornstarch with 1/4 cup water, and add to gravy. Cook and stir until thickened and bubbly.
Serve sausage with mashed potatoes, gravy, and peas.