Herbed Oyster Mushroom Roast

Name: Herbed Oyster Mushroom Roast
Ingredients: 2 pounds oyster mushrooms; 1/2 teaspoon to 1 teaspoon kosher salt, per your taste; 1/2 teaspoon freshly ground black pepper; 1/2 teaspoon sweet paprika; 1 teaspoon smoked paprika; 1 teaspoon tomato paste; 1 teaspoon Dijon mustard; 1 teaspoon soy sauce; 2 teaspoons chopped fresh rosemary; 2 tablespoons light brown sugar; 2 tablespoons water; 3 cloves of garlic, minced; 1/3 cup extra-virgin olive oil, plus more for drizzling on top; 2 to 3 medium yellow onions; 5 sprigs fresh rosemary