Craft the Nutrient-Dense Filling: In a food processor, combine lentils, toasted walnuts, whole-wheat panko, 2 cloves garlic, smoked paprika, cumin, and optional red pepper flakes. Pulse to a thick, coarse paste. Transfer to a small bowl.
Ignite the Chermoula Base: Wipe the processor. Add cilantro, parsley, remaining 2 cloves garlic, lemon juice, lemon zest, and the salt. Pulse until finely minced.
Emulsify the Marinade: With the processor running, slowly stream in 5 tablespoons olive oil until emulsified. Reserve 1/3 cup chermoula for serving; transfer the remainder to a large mixing bowl.
Assemble the Sardine Medallions: Pat sardines dry. Add to the bowl with chermoula and toss gently to coat. Lay one fillet skin-side down; spread about 1–2 tablespoons filling over the flesh. Top with a second fillet skin-side up to form a medallion. Repeat to make about 6 medallions.
Flavor Infusion: Arrange medallions on a plate, cover, and refrigerate 30 minutes (up to 4 hours).
Prepare the Grill/Broiler: Preheat a gas grill to high or prepare a hot charcoal fire for direct heat; clean grates. If using a grill basket, brush with the remaining 1 tablespoon olive oil. For oven broiling, position a rack 6 inches from the element and preheat broiler on high.
Grill to Smoky Perfection: Place medallions in the basket or directly on grates. Grill 3–5 minutes per side, flipping once, until skin is blistered and the fish is opaque; target 60–63°C (140–145°F) internal temperature or flakes easily with a fork. For broiling, cook 4–6 minutes total, flipping once, until lightly charred and just cooked through.
Final Presentation: Transfer to a platter. Whisk the reserved 1/3 cup chermoula and drizzle over the hot fish. Serve immediately with lemon wedges.