Pot Roast Sliders with Gravy

Name: Pot Roast Sliders with Gravy
Ingredients: One 4-pound chuck roast; 1 1/2 teaspoons kosher salt; 1/2 teaspoon freshly ground black pepper; 3 cloves garlic, minced; 2 tablespoons vegetable oil; 2 1/2 to 3 cups beef broth; 1 medium sweet onion, sliced; 1 green bell pepper, sliced; 2 tablespoons all-purpose flour; 1/3 cup mayonnaise, such as Duke's; 3 tablespoons prepared horseradish; 24 to 30 dill pickle slices (crinkle cut chips); 24 slider buns, halved, such as Martin's Potato Rolls