Slice 1/4 inch from stem end of each tomato; discard stem ends. Carefully scoop out pulp, leaving shells intact. Chop pulp; set aside. Invert tomato shells on paper towels to drain.
Coat a medium nonstick skillet with cooking spray; add oil. Place over medium-high heat until hot. Add shallot; saute 3 minutes. Add reserved tomato pulp and spinach; cook 3 minutes, stirring frequently.
Transfer vegetable mixture to a bowl. Stir in rice, 1 tablespoon cheese, mayonnaise, and lemon juice. Spoon vegetable mixture evenly into tomato shells. Place tomatoes in a small shallow casserole coated with cooking spray. Bake at 325° for 15 minutes; sprinkle with remaining 1 tablespoon cheese. Bake an additional 3 minutes or until cheese melts.