Grilled Meatball Sandwich

Name: Grilled Meatball Sandwich
Ingredients: 1/2 cup walnut pieces; 1 garlic clove, minced; 2 cups packed arugula leaves; 1/2 cup grated Parmigiano-Reggiano cheese; 1 tsp. kosher salt, plus more, to taste; 1 cup extra-virgin olive oil; 2 lb. ground beef; 1/2 cup ricotta cheese; 1/4 cup grated Parmigiano-Reggiano cheese; 1/4 cup panko bread crumbs; 4 eggs, lightly beaten; 2 Tbs. minced fresh flat-leaf parsley; 1 tsp. red pepper flakes; 2 tsp. kosher salt; Oil for grill; 1 large baguette; 8 oz. thinly sliced provolone cheese