Preheat the oven to 325°. In a Dutch oven, heat the butter over medium-high heat. Add onion; cook and stir until tender, 2-3 minutes. Stir in flour until blended; gradually whisk in milk. Bring to a boil, stirring constantly; cook and stir until thickened, 1-2 minutes. Stir in cheese and salt until cheese is melted., Remove from heat. Gradually whisk in eggs. Stir in vegetable blend. Transfer to a greased 8-in.-square baking dish. Bake, uncovered, until set, 50-60 minutes. Let stand 10 minutes before serving.