In a large bowl combine green beans, mustard, cayenne pepper, paprika, cumin, chili powder, garlic and salt; toss to coat beans
Heat in a large skillet (preferably cast iron) over high heat until smoking
Add the beans, being careful not to crowd the pan too much (cook in two batches, if necessary). Cook beans, without disturbing, until charred patches appear, about 5 minutes.
Toss beans and cook 5 minutes or more.
Transfer to a bowl or plate and serve immediately with Spicy Dipping Sauce
Spicy Dipping Sauce
Mix mayonnaise, cayenne pepper, mustard, capers, garlic, parsley and lemon juice until combined