Chop all veggies and ham. Then place the onions, bell peppers, and garlic in a small nonstick skillet and sear over medium to soften, 3-5 minutes. Remove from heat.
Meanwhile, place an 18-inch piece of parchment paper on the countertop. Place the two rolled-out pie crusts on the parchment, overlapping in the center, to create one long piece of crust, about 17- to 18-inches long. Press the overlapped portion together, then move into a large 6-quart oval slow cooker. Gently fit the crust down into the creases of the slow cooker, and press the excess crust against the sides. Crimp the edges, making sure the crust goes 2 1/2 to 3 inches up the sides.
In a large mixing bowl, whisk the eggs, cream, ground mustard, and 1/2 teaspoon salt together until smooth and frothy. Whisk in black pepper to taste. Then stir all the veggies, ham and cheese into the egg mixture. Pour the mixture into the prepared crust.
Press the sides of the crust against the crock again to make sure they are secure. Then place a long piece of paper towel over the slow cooker and cover with the lid. Pull the paper towel edges tight so it doesn't sag. (The paper towel with absorb condensation.) Turn the slow cooker on high for 3-4 hours, until the edges of the crust are brown and the center of the quiche is set. Lift the entire quiche out of the slow cooker by the edges of the parchment paper. Cool for 10 minutes, then cut and serve.