Line a 15x10 inch baking with foil. Place the onions, shallots, and garlic in the pan. Drizzle with olive oil. Roast for 40 minutes or until tender and lightly charred
In a food processor or blender combine the vegetables from the pan and 1/2 cup of the chicken broth. Cover and process or blend until smooth; set aside.
In a medium saucepan melt butter. Stir in flour, chives, lemon zest, salt, and pepper. Add milk all at once. Cook and stir until slightly thickened and bubbly. Cook and stir 1 minute more.
Stir in the blended onion mixture, and the remaining 3/4 cup broth. Cook and stir until heated through. If necessary, stir in additional milk to reach desired consistency. Season to taste with additional salt and pepper.