Bring 3 quarts water to a boil in a Dutch oven. Add lasagna noodles; cook 11 minutes or just until tender. Drain well. Cut in half crosswise, and set aside.
Cook onion in hot oil in pan over medium-high heat 5 minutes or until tender. Add vegetable mix, 1/2 teaspoon salt, pepper, and 1/2 cup water. Bring to a boil; cover, reduce heat, and simmer 7 minutes or until tender. Uncover; increase heat to high, and cook until liquid evaporates. Set aside.
Combine flour, milk, and remaining 3/4 teaspoon salt in a medium saucepan; beat with a whisk until smooth. Cook over medium-high heat, whisking constantly, until thickened and bubbly. Cook an additional 2 minutes. Remove from heat, and stir in 3/4 cup plus 2 tablespoons Gruyère cheese. Stir 1 1/2 cups cheese sauce into vegetable mixture.
Place 5 lasagna noodle halves in bottom of a 9-inch square baking dish coated with cooking spray. Top with one-third of vegetable mixture and 5 lasagna noodles. Repeat process with remaining vegetable mixture and lasagna noodles, ending with lasagna noodles. Pour remaining cheese sauce over top of lasagna noodles and sprinkle with remaining 2 tablespoons Gruyère cheese. Bake, uncovered, at 375° for 20 minutes or until golden. Let stand 15 minutes.