Preheat the oven to 500°F Coat a medium baking dish with cooking spray. Coat a baking sheet with cooking spray.
Place the buttermilk in a shallow bowl. Place the bread crumbs in another shallow bowl. Dip the onion rings into the buttermilk, then dredge in the bread crumbs and place on the baking sheet. Coat lightly with cooking spray. Bake for 20 minutes, or until tender and golden brown.
Meanwhile, coat a medium saucepan with cooking spray. Set over medium heat. Add the mushrooms, chopped onion, thyme, and salt. Coat with cooking spray. Cook, stirring occasionally, for 4 to 5 minutes, or until the mushrooms give off liquid.
Sprinkle with the flour. Cook, stirring, for 1 minute.
Add the milk. Cook, stirring constantly, for 3 to 4 minutes, or until thickened.
Add the green beans. Stir to mix
Reduce the oven temperature to 400°F Pour the bean mixture into the prepared baking dish. Scatter the onion rings over the top. Bake for 25 to 30 minutes, or until hot and bubbly.