1. Bring water to a boil in a medium saucepan. Whisk in grits, reduce heat to medium low and simmer, whisking occasionally, until thick, 4 to 5 minutes. Remove from heat; stir in cheddar, 1/8 teaspoon salt and 1/4 teaspoon pepper and cover to keep warm.
2. Sprinkle tomatoes and mushrooms with remaining salt and pepper. Coat a large nonstick skillet with cooking spray and heat over medium-high heat. Place mushrooms and tomatoes cut side down in skillet and cook until slightly soft, 4 to 5 minutes. Flip and cook 3 to 4 minutes more. Transfer vegetables to a plate.
3. Wipe skillet clean, coat with cooking spray and cook bacon until crisp, 1 to 2 minutes a side. Divide grits, tomatoes and mushrooms among four plates and serve with bacon on the side. Garnish with parsley if desired.