1.Heat oven to 400°F. Grease 13x9-inch baking pan.
2.In large saucepan, bring water to a boil; gradually stir in salt and grits. Reduce heat; simmer, stirring occasionally, 25 to 30 minutes or until thickened. Stir in 1 1/3 cups cheese.
3.In bowl, mix SPAM® Classic and corn.
4.In separate bowl, mix mushrooms and onions.
5.Spread 1/3 of grits into bottom of baking pan. Top with 1/3 each of spaghetti sauce and SPAM® mixture. Top with 1/3 each of grits and spaghetti sauce and the mushroom mixture. Sprinkle with remaining 1/3 of grits; spoon remaining 1/3 of spaghetti sauce on top. Sprinkle with remaining 2/3 cup cheese.
6.Bake casserole 25 to 30 minutes or until bubbly.