Position oven rack just below center of oven; heat oven to 450°F. Place beef, fat-side up, in shallow roasting pan.
In small bowl, combine garlic, herbs de Provence, salt and pepper; mix well. Spread 2 tablespoons mustard over beef; pat garlic-herb mixture onto mustard. Roast at 450°F for 25 minutes. Reduce temperature to 350°F; roast 1 hour 20 minutes longer until instant-read thermometer registers 130°F for medium-rare.
While beef is roasting, in bowl, mix sour cream, remaining mustard, salt and 2 tablespoons water. Refrigerate.
Remove beef from oven; let rest 15 minutes before carving. Serve with sour cream sauce and stuffed mushrooms, cheddar-cheese mashed potatoes and steamed broccoli on the side.
Heat purchased frozen stuffed mushrooms and lightly sprinkle with Parmesan.
Prepare 1 package (22 ounces) frozen mashed potatoes as package label directs, substituting 2-1/2 cups fat-free half and half for the milk. Stir in 1 cup shredded cheddar cheese before serving.
Steam one bunch broccoli, cut into florets; serve with a squeeze of lemon juice and chopped jarred pimientos.