Fill up a large bowl of water and a few handfuls of ice, set aside.
Bring a large pot of water to a rolling boil. Add a heavy pinch of salt, the green beans, and the bell peppers. Stir to dissolve the salt and cook the vegetables until bright and still crisp, just a few minutes. With a sieve or slotted spoon, remove the vegetables to the bowl of ice water to stop them from over cooking. Once the vegetables are cooled remove them to a paper towel-lined baking sheet and pat dry.
To prepare the mushrooms: In a small saucepot on medium heat add 2 cups of water, the vinegar, mushrooms, garlic, thyme, and 1 teaspoon salt. Stir to dissolve the salt, cover and cook on medium heat for 15 minutes. Turn off the heat and let it rest covered for 15 minutes more.
To make the vinaigrette: Whisk together the mustard, 4 tablespoons of the liquid the mushrooms are soaking in, and olive oil in a small bowl. Season with some salt and a grind or two of pepper.
Shake the excess liquid off the mushrooms and add them to a large bowl along with the cooked vegetables and the vinaigrette. Toss and serve with fried onions sprinkled over the top.