In a large pot or skillet, melt the butter on medium heat. Add onions and garlic, and saute until the onions sweat and turn soft. Add cumin and coriander and stir to coat. Let this cook for 1-2 minutes to bloom the spices, then remove from heat. Cool for a minute or two, then stir in the flour, coating everything evenly. Return to medium heat and cook, stirring, for about two minutes. (At this point, you have created a spice-enhanced roux, and are browning the flour slightly.) Slowly pour in milk, stirring often, until everything is well mixed. Add the mushrooms, reduce heat to medium low, and cook for 6-7 minutes, again stirring often. As the mushrooms cook, they will release moisture, and a sauce will form. Keep stirring gently to ensure the mixture doesn't burn on the bottom. (If necessary, and a few tablespoons of milk to create cooking moisture.)  When the mushrooms are fully cooked, add the white pepper, stir to combine completely, and cook another minute or so. Taste the dish, and add salt as needed. Serve as a side dish with your favorite Indian main!