Gather ingredients so that you have everything close at hand on the counter or table. If using fresh green beans, trim ends as needed. Slice button mushrooms. Coarsely chop or mince the sweet onions.
If using frozen green beans, cook in microwave according to package directions. If using fresh green beans, place in a large microwavable dish. Add in 2 tablespoons water to create steam. Cook on high for 10-12 minutes until crisp tender. Transfer green beans to a large serving dish.
Meanwhile in a large skillet, heat olive oil and butter. Add sliced mushrooms and chopped onions and saute until tender 4-6 minutes.
Mix flour into melted fat to create a roux. Slowly add milk, stirring until it thickens 3-5 minutes. Season with dried thyme, kosher salt, and freshly ground black pepper.
Spoon sauce over green beans and top with slivered almonds. Serves 10-12.
Notes
*Unless otherwise indicated, all butter used on this recipe site is salted.