Home on the Range Bison Steak

Name: Home on the Range Bison Steak
Ingredients: 2 pounds bison flank steak, in 2 or 3 pieces (or beef if you can not find bison); kosher salt; 1/4 cup brown mustard; 1/4 cup minus two tablespoons of creme de cassis; 2 tablespoons cider vinegar; 1 tablespoon bourbon, Maker's Mark or other type; 2 tablespoons grapeseed oil; spray of grapeseed oil; 1 red onion, chopped into rings; 1 stalk of rhubarb, sliced into 3/4 inch chunks; @ 1/2 teaspoons crushed pink and black peppercorns; fresh mint sprigs, divided