Don't overcook the beans: I only blanch them for 2 or 3 minutes because I like the green beans to keep some bite. Remember that they'll continue cooking for another 40 minutes or so in the oven.
Shred the cheese yourself: Freshly shredded Monterey Jack melts smoothly into the mushroom sauce and doesn't contain the anti-caking ingredients commonly used on pre-shredded cheese.
Drain the beans well: Extra water on the beans can thin the mushroom sauce, so give them a good shake in the colander after blanching.
Save most of the onions for the top: I mix a smaller amount into the casserole for flavor and add the rest toward the end. That way you still get plenty of crispy onions instead of having them all soften in the sauce.
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