This recipe is originally from cookbook author Anne Willan who calls them "burnt beans." It deals with the problem of mushy green beans, which is what happens when you cook any frozen green bean--even those you've picked, blanched and frozen straight from your garden. But in this recipe, the beans slow cook until crispy. Don't fret the butter.
Heat a large cast-iron pan or skillet over medium-high heat and add the butter.
When the butter foams, add the green beans, stir to coat them in the butter and leave them to heat until they're no longer frost coated, about 5 minutes.
Add the salt and pepper. Reduce the heat to medium-low so that you can still hear them sizzling and stir the beans intermittently until they shrink, wrinkle and about half of them are darkly colored, about 30 minutes more.
Add the shallots and mushrooms for the last 10 to 15 minutes of cooking until the mushrooms brown and the shallots soften.
Taste for seasoning before serving.
For some reason, frozen green beans come in many sizes, from 10 ounces up 19 ounces. I've written this recipe for 1 pound of frozen green beans--home grown or commercial--so that you can readily scale it to whatever amount of green beans you have on hand.