Craft the Nutrient-Dense Filling: In the bowl of a food processor, combine the cooked lentils, toasted walnuts, whole-wheat panko, 2 cloves of garlic, smoked paprika, cumin, and optional red pepper flakes. Pulse until the mixture forms a thick, coarse paste. Scrape down the sides as needed. Transfer the filling to a small bowl and set aside.
Ignite the Chermoula Base: Wipe out the food processor bowl. Add the cilantro, parsley, the remaining 2 cloves of garlic, lemon juice, and lemon zest. Pulse until the herbs and garlic are finely minced.
Emulsify the Marinade: With the food processor running, slowly stream in 5 tablespoons of the extra virgin olive oil until a bright, emulsified marinade forms. Measure out and reserve 1/3 cup of this chermoula in a separate small bowl for serving later. Pour the remaining chermoula into a large mixing bowl.
Assemble the Sardine Medallions: Gently add the butterflied sardines to the large bowl with the chermoula and toss carefully to coat each fillet. Lay one sardine fillet on a clean plate, skin-side down. Spread approximately 1 to 2 teaspoons of the walnut-lentil filling evenly over the flesh. Top with a second sardine fillet, skin-side up, to create a "medallion." Repeat with the remaining sardines and filling.
The Flavor Infusion: Arrange the assembled medallions on a plate, cover loosely, and refrigerate for at least 30 minutes (or up to 4 hours) to allow the flavors to meld.
Prepare the Grill for a Perfect Sear: Preheat your gas grill to high or prepare a charcoal grill for direct, high-heat cooking. Thoroughly clean the grates. If using a grill basket, brush it lightly with the remaining 1 tablespoon of olive oil. If using an oven, position a rack 6 inches from the broiler and preheat the broiler on high.
Grill to Smoky Perfection: Carefully arrange the sardine medallions in the oiled grill basket or directly on the grates. Grill for 3-5 minutes per side, flipping once, until the skin is blistered and crisp, and the fish is opaque and cooked through. If broiling, arrange on a foil-lined sheet pan and broil for 4-6 minutes, flipping once halfway, until lightly charred.
Final Presentation: Transfer the cooked medallions to a platter. Whisk the reserved 1/3 cup of chermoula to re-combine, and drizzle it artfully over the hot fish. Serve immediately with fresh lemon wedges on the side.