Step 1

In a medium-sized bowl stir together the milk and vinegar. Add onion rings and use a spoon to stir and coat them. Set aside for onions to marinate for approximately 30 minutes.

Step 2

On a large plate, stir together flour, salt, and pepper.

Step 3

Use tongs to pull a small batch of the onions from the milk mixture, dangling them over the bowl to let excess liquid drip off. Transfer the onions to the flour mix. Repeat until all onions have been transferred. Use tongs to toss and coat. Reserve the milk mixture and excess flour for the casserole.

Step 4

Cover a plate with 2 to 3 paper towels. Heat 1 tablespoon oil in a large skillet over medium heat. Once the oil is heated, add some of the battered onions in an even layer across the pan with minimal overlapping. Cook 2 to 3 minutes on each side, using tongs to gently toss. The onions should begin to turn a golden brown and become crispy on the edges. Transfer cooked onions to a paper towel-lined plate. Repeat this step with the remaining onions.

Step 5

In a skillet over medium heat, add vegan butter and cook until melted. Add the mushrooms and cook until tender, about 3 minutes. Add minced garlic. Stir and cook for about 30 seconds.

Step 6

Stir together the milk mixture from the onions above, with 1 cup water, and bouillon. Use any leftover flour from the fried onions for a total of 3 tablespoons flour to the milk mixture (supplement with additional flour as needed). Stir to remove any lumps. Pour this over the contents in the skillet and stir to combine.

Step 7

Add the green beans, stir to coat, and then bring it to a simmer. Add vegan cheese and stir until it melts. Then reduce heat to medium-low and cook for approximately 5 minutes, until the green beans are no longer frozen and slightly tender, but still a little crisp and the sauce has thickened. If the sauce does not thicken, sprinkle another tablespoon of flour over the top and use a spatula to stir it in.

Step 8

While the green beans simmer, preheat the oven to 375°F/191°C.

Step 9

Pour the green bean mixture into a 9" square baking dish or casserole dish. Add about 1/2 of the fried onions and stir them throughout. Equally, distribute remaining fried onions over the top. Sprinkle with vegan parmesan. Bake for 20 to 25 minutes.

Step 10

When done, remove from oven and set aside to cool for 5–10 minutes before serving. Cover and refrigerate any leftovers not eaten right away. These will keep in the fridge for up to 5 days.