Start by caramelizing onions. Heat a skillet over medium heat. Once hot, add one tablespoon of olive oil. Add sliced onions to the hot oil and sprinkle with ⅛ teaspoon salt. Saute for 8-10 minutes over medium heat, until softened. Reduce heat to medium low. Cook onions for another 40-45 minutes, stirring only every 10 minutes. If onions start to brown too quickly, splash with a teaspoon of water. The onions are ready when deep gold and very soft.
Preheat the oven to 400 degrees F.
Meanwhile, prep remaining ingredients. Fill a 3 quart pot half full with water. Add ½ teaspoon coarse salt. Bring to a boil over high heat. Add green beans and cook until al dente, about 5-7 minutes. Drain and set aside.
To make the cream sauce, heat a large skillet over medium-high heat. Add two tablespoons of butter. Once melted, add the chopped mushrooms. Cook for 6-8 minutes, until softened. Add the minced garlic and cook until golden and fragrant, about 1 minute. Sprinkle in the all-purpose flour. Cook for another minute, stirring frequently to prevent burning.
Using a whisk, slowly stir vegetable broth into the flour and garlic. Continue whisking while you slowly pour in the milk. Sprinkle with ½ teaspoon salt and pepper. Once the mixture starts bubbling over medium-high heat, reduce to low. Simmer until thickened, stirring occasionally, about 10 minutes. It should be thick enough to coat the back of a spoon without falling off.
Lightly coat a 2 quart casserole dish with non-stick spray or oil. Evenly layer the cooked green beans across the bottom. Cover with the caramelized onions (about 1 cup). Pour the sauce over the green beans and onions. Sprinkle the top with french fried onions.
Cover with tin foil. Bake at 400 degrees for 25 minutes. Take tin foil off for the last 10 minutes. The casserole is ready when it is bubbling and the fried onions are golden brown. Enjoy!
Green beans: Frozen green beans can also be used. Cook according to package instructions before adding to the casserole dish.Gluten-free option: Use a gluten-free all-purpose flour mix in place of standard all-purpose flour.Vegan options: Use unsweetened, unflavored soy or almond milk and vegan butter or olive oil in place of whole milk and dairy butter.Make-ahead options: Make the casserole according to the recipe instructions but don't add the french onions. Cover and store in the fridge for up to 3 days. Remove from the fridge and let come to room temp while the oven is preheating. Add the fried onion topping and cook according to recipe instructions.