Preheat the oven or a toaster oven to 400 degrees F.
For the green beans: Place the green beans on a sheet pan. Add the mushrooms, olive oil and capers, sprinkle with 1/4 teaspoon salt and toss to coat. Roast the green beans until tender and the skins blister, 8 to 10 minutes depending on the thickness of the green beans.
For the goat cheese sauce: Meanwhile, add the butter to a small saute pan over medium heat and cook until it starts to bubble and brown. Add the flour and stir to cook the flour, 1 to 3 minutes. Slowly whisk in the milk. Continue to cook until the mixture simmers and thickens, about 10 minutes. Add the garlic powder and pepper. Remove from the heat and add 3 ounces of the goat cheese. Taste and season with salt.
For the garnish: When the green beans are tender, pour the goat cheese sauce on the bottom of a platter. Top with the roasted green beans and sprinkle with the remaining goat cheese and crispy fried onions. Serve immediately.