Coat a 6-quart slow cooker with cooking spray. Add potatoes, zucchini and thyme.
Coat a large nonstick skillet with cooking spray, place over medium high heat until hot. Add the onions, coat with cooking spray; cook 4 minutes or until translucent, stirring frequently. Add the bell peppers to the onions in the skillet, coat with cooking spray, cook 4 minutes or until onions are brown on edges, stirring frequently.
Pour the water into the skillet and stir until well blended and beginning to boil. Pour onion mixture evenly over the potatoes. Do not stir. Cover and cook on high setting 3 1/2 hours or until potatoes are tender when pierced with a fork.
Combine the milk and cornstarch in a small bowl; stir until cornstarch is completely dissolved. Gently stir into the potato mixture with 1 cup of the cheese, the butter, salt and pepper. Cover and cook on high setting 10 minutes to thicken slightly.
Spoon the potato mixture into a large pasta bowl or 13-inch by 9-inch casserole dish. Sprinkle with the remaining 3/4 cup cheese.