Wrap potatoes in foil; bake at 400°F 1 hour. Let cool; peel and mash.
Remove stems from spinach, and wash leaves. Place large Dutch oven over medium heat and add spinach; cover and cook 2 minutes or until spinach wilts (do not add water). Dry spinach between paper towels. Chop into fine pieces.
Combine mashed potatoes, chopped spinach, 1/2 cup flour, ricotta cheese, salt, pepper, nutmeg and egg; stir to form soft dough. Turn dough out onto well-floured surface; knead in remaining flour. Divide dough into 4 equal portions, shaping each into 12-inch rope. Cut each rope into 12 (1- inch) pieces; roll each piece into a ball. Drag tines of a fork through half of each ball, forming a concave gnocchi shape. Place on a baking sheet coated with cooking spray; set aside.
Bring 14 cups of water to a boil in Dutch oven. Add half of the gnocchi, cook 1-1/2 minutes. Remove gnocchi with slotted spoon to colander. Repeat with remaining gnocchi.
Melt butter in large skillet over medium heat and cook until nutty-brown color. Add tomatoes and garlic; sauté 2 minutes. Remove from heat; stir in vinegar.
Place gnocchi in 2-quart baking dish coated with cooking spray. Spoon tomato mixture over gnocchi; sprinkle with BelGioioso Asiago Cheese. Bake at 350°F 30 minutes. Sprinkle with chives.
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