Note: Recipe can be made-ahead through step 3 and refrigerated until ready to serve. To serve, combine beans, mushrooms, and onions in a large skillet and cook over medium-high heat, stirring, until hot.
- 4 tablespoons unsalted butter or extra-virgin olive oil, divided
- 1 pound cipollini onions, trimmed and peeled
- Kosher salt and freshly ground black pepper
- 2 pounds green beans, trimmed
- 2 tablespoons canola or vegetable oil
- 1 pound button mushrooms, washed, trimmed, and cut into quarters
- 1 medium shallot, finely chopped
- 4 medium cloves garlic, minced (about 4 teaspoons)
- 1 teaspoon picked fresh thyme leaves
- 1 teaspoon soy sauce
- 1 teaspoon juice from 1 lemon
Melt 3 tablespoons butter (or heat olive oil) in a large non-stick or cast iron skillet over medium heat. Add cipollini onions, season well with salt and pepper, reduce heat to low, and cook, turning occasionally, until onions are a deep, caramel brown, about 45 minutes total.
Meanwhile, bring a large pot of salted water to a boil. Add beans and cook until tender-crisp, about 3 minutes. Drain and run under cool running water until at cold. Set aside.
Heat oil in a large saucepan over high heat until lightly smoking. Add mushrooms and cook, stirring occasionally, until they've released all their liquid and are browned, about 10 minutes total, reducing heat if oil starts to smoke excessively. Season to taste with salt and pepper. Add shallots, garlic, thyme, and remaining tablespoon butter (or olive oil) and cook, stirring, until fragrant, about 30 seconds. Add soy sauce and toss to combine.
Add green beans, onions, and lemon juice to mushrooms and toss to reheat and combine. Serve immediately.