To make the arugula pesto, in a food processor, combine the walnuts, garlic, arugula, Parmigiano-Reggiano and the 1 tsp. salt and pulse to blend. With the machine running, pour in the olive oil through the feed tube in a slow, steady stream and process until smooth, stopping to scrape down the sides of the bowl as needed. Taste and adjust the seasonings with salt. Set aside.
In a large bowl, combine the ground beef, ricotta, Parmigiano-Reggiano, bread crumbs, eggs, parsley, garlic, red pepper flakes and salt.
Mix gently just until combined; you don't want to overwork the meat. Form the mixture into 12 meatballs about the size of golf balls, putting them on a lightly oiled baking sheet as you work. Set aside at room temperature.
Build a hot fire in a charcoal grill or preheat a gas grill to high. Using a grill brush, scrape the heated grill rack clean. Rub the rack with oil.
Arrange the meatballs on the grill rack without crowding. Using tongs, grill until the meatballs are browned evenly on all sides and cooked to medium, 8 to 10 minutes total, depending on the grill temperature. Move any meatballs to a cooler area of the grill if they threaten to overbrown.
Transfer to a platter or a clean baking sheet as they are finished.
Let rest for 5 minutes while you assemble the sandwiches.
Cut the baguette crosswise into 4 pieces and split each piece horizontally.
Lay the pieces, cut side down, on the grill. Toast until golden brown, about 3 minutes.
Lay the provolone slices on half of the baguette pieces.
Place 3 meatballs on top of the cheese-lined baguette slices.
Garnish with the pesto, cover the sandwiches with the remaining baguette pieces and serve immediately.
