Preheat oven to 425 degrees. In a food processor, pulse bread until pea-size crumbs form. Transfer to a baking sheet and bake until crisp, about 5 minutes. Combine fennel bulb, fennel fronds, onion, apple, sage, thyme, 1 tsp. kosher salt, and ground black pepper, to taste, in a food processor, until finely chopped.
In a medium-size skillet, heat 1 Tbsp. olive oil over medium heat. Add fennel mixture, and cook until the fennel and onions are soft, about 8 to 10 minutes. Transfer to a bowl and cool to room temperature. When cool, stir in 1 Tbsp. olive oil and reserved breadcrumbs.
Season pork with 1 tsp. kosher salt. Spread fennel mixture over the top and sides of the pork, and put in a roasting pan or rimmed baking sheet. Place in the oven and roast until the internal temperature reads 140 degrees on an instant-read thermometer, about 1 hour, 20 minutes. If the crust gets too dark before the pork is fully cooked, cover lightly with foil. Remove from oven, tent loosely with foil, and let rest for at least 15 minutes before slicing.