Mince the anchovies, capers, garlic and red pepper flakes together on a cutting board, then sprinkle with 1 teaspoon salt and mash into a paste using the flat side of a chef's knife. Transfer to a food processor along with the parsley, mint and marjoram. With the motor running, drizzle in the olive oil and process until smooth; transfer to a medium bowl.
Toast the fennel seeds in a skillet over medium heat, 3 to 4 minutes. Transfer to a cutting board and crush with the bottom of a small skillet. Combine with 1/4 cup of the herb mixture and the herbes de Provence in a small bowl. Put the fish fillets in a shallow dish and brush with the fennel seed-herb mixture. Cover and refrigerate 2 to 4 hours.
Meanwhile, add the lemon juice, bell pepper and cucumbers to the remaining herb mixture. Cover and refrigerate until ready to serve.
Heat 2 large nonstick skillets over medium-high heat. Brush the excess marinade off the fish. Working in batches, add the fish to the skillets, skin-side up, and cook until browned on the bottom, about 4 minutes. Flip the fillets, pressing with a spatula to sear the skin, and cook 2 more minutes. Serve with the herb salsa and lemon wedges.