Preheat oven to 375°F. Carefully pierce squash all over with a small sharp knife. (If too difficult to pierce, microwave squash on HIGH 3 to 4 minutes or until tender enough to pierce.) Place squash on foil-lined shallow baking pan.
Bake 1 hour 20 minutes or until tender when pierced with a knife, turning halfway through cooking. Let cool slightly, about 20 minutes. Meanwhile, beat Seasoning, milk and egg in large bowl until well blended. Stir in tomatoes. Set aside.
Cut squash in half lengthwise; remove seeds. Using fingers or a fork, shred squash flesh into long thin strands into the bowl with the egg mixture; stir to blend. Place squash shells, cut side up, back on the baking pan. Set aside.
Melt butter in large skillet on medium-high heat. Add onion; cook and stir until translucent, about 3 to 5 minutes. Add spinach; cook and stir until wilted, about 2 minutes. Stir into squash mixture. Fill shells evenly with squash mixture. Top with cheese.
Bake 30 minutes or until bubbly and lightly browned.