Mushrooms with Béarnaise Yogurt

Name: Mushrooms with Béarnaise Yogurt
Ingredients: 1 shallot, finely chopped; 2 sprigs tarragon, plus 2 teaspoons chopped; ½ teaspoon black peppercorns, crushed; 1 tablespoon white wine vinegar; 1 cup plain whole-fat Greek yogurt; 2 tablespoons fresh lemon juice; 1 tablespoon olive oil; Kosher salt; 1 tablespoon coriander seeds; 1 teaspoon kosher salt; 1 teaspoon mustard powder; 1 teaspoon paprika; 12 ounces maitake or oyster mushrooms, torn into large pieces; 2 tablespoons olive oil; Torn tarragon sprigs (for serving); A spice mill or a mortar and pestle