For dairy-free, use plant-based milk and butter; for gluten-free, use a 1:1 gluten-free flour blend for the roux. If mushrooms are not preferred, finely diced leeks or shallots (about 1 cup, 120g) can be sautéed instead. Opt for organic vegetables and low-sodium broth for higher quality and controlled sodium. For a protein boost, stir in 2-3 tablespoons of nutritional yeast to the sauce. Fresh green beans offer superior texture and flavor, though frozen can be used. Do not overcook green beans initially, as they will soften further in the oven. Avoid overcrowding the skillet to ensure proper browning of mushrooms. Cook the roux gently for 1-2 minutes to prevent burning. For an extra rich casserole, use heavy cream instead of milk; for a lighter version, use skim milk or unsweetened almond milk. For extra crunch, add another 0.5 cup (35g) of crispy onions during the last 10 minutes of baking.