• Boil beans in a medium-sized pot of salted water (make sure there's enough water to cover the beans) until just about cooked through and beginning to soften (about 6-8 minutes -- be careful not to over cook or they will be too soft). 
• Drain in a colander and immerse them in a large bowl of water with ice to stop the cooking process. Set aside.
• While beans are blanching, prepare the crisp onion topping.

CRISP ONION TOPPING:

• Melt butter in a large pan or skillet over medium-high heat. Fry onion rings until transparent and fragrant. 
• Add the breadcrumbs and cook, while stirring, until breadcrumbs are golden brown and toasted. 
• Then add in the parmesan cheese and continue cooking for a further minute or two until the cheese just begins to melt. Transfer to a bowl and set aside.

SAUCE:

• Melt the 2 tablespoons of butter in the same pan or skillet over medium-high heat. Fry the diced onion until transparent.
•  Add the mushrooms, salt and pepper and cook, stirring occasionally. (The mushrooms will begin to release water after about 4 minutes.) Add in the garlic and continue to cook for a further minute until fragrant.
• Mix the flour through the mushrooms and allow to cook for a further minute. 
• Pour in the broth and simmer for 2 minutes, until just beginning to thicken. 
• Reduce heat to low and pour in the milk (or cream), while stirring. Cook until the sauce thickens (about 1-2 minutes).
• Add in the beans and cheddar cheese. Allow the beans to cook in the sauce to your desired doneness while the cheese melts through the sauce, stirring occasionally to prevent sticking or burning on the bottom of the pan (about 8-10 minutes).
• Top with the crisp onion mixture and serve immediately.