To prepare crust, lightly spoon flour into a dry measuring cup; level with a knife. Place flour, 1/2 teaspoon salt, and peppers in a food processor; pulse 2 times. Add chilled butter; pulse 4 times or until mixture resembles coarse meal. Place mixture in a bowl. Sprinkle surface with ice water, 1 tablespoon at a time; toss until moist and crumbly (do not form a ball). Place mixture on a lightly floured surface, and knead lightly 2 to 3 times. Press mixture into a 4-inch circle on plastic wrap; cover. Chill 30 minutes.
Preheat oven to 450°.
Slightly overlap 2 sheets of plastic wrap on a slightly damp surface. Unwrap and place chilled dough on plastic wrap. Cover dough with 2 additional sheets of overlapping plastic wrap. Roll dough, still covered, into a 10-inch circle. Place dough in freezer 5 minutes or until plastic wrap can be easily removed.
Remove top sheets of plastic wrap; fit dough, plastic wrap side up, into a 9-inch round removable-bottom tart pan coated with cooking spray. Remove the remaining plastic wrap. Press dough against bottom and sides of pan. Line bottom of dough with a piece of parchment paper; arrange pie weights or dried beans on paper. Bake at 450° for 20 minutes or until edge is lightly browned. Carefully remove pie weights and paper; cool on wire rack. Reduce oven to 400°.
To prepare filling, bring a large saucepan of water to a boil. Add potatoes; cook 20 minutes or until tender. Cool; cut into 1/4-inch-thick slices. Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add chopped onion; sauté 5 minutes or until tender. Stir in 1/4 cup water. Add fresh spinach; cover and cook for 3 minutes or until the spinach wilts, stirring occasionally. Remove from heat; cool.
Spoon half of spinach mixture into cooled crust. Arrange half of potato slices over spinach. Sprinkle with 1/4 teaspoon salt and 1/4 cup cheese. Repeat procedure with remaining spinach, potato, and salt. Pour egg substitute over mixture. Sprinkle with remaining 1/4 cup cheese. Bake at 400° for 25 minutes or until mixture is set and cheese is golden brown. Cool 15 minutes on a wire rack.