In a large bowl, combine 3 tablespoons of oil, lemon juice, herbs, salt, pepper, and shrimp. Cover and refrigerate for 20 to 30 minutes.
Soak skewers in warm water at least 20 minutes while shrimp is marinating.
Heat remaining oil in a large skillet over low heat and wilt spinach with fennel seeds, about 3 to 4 minutes (do in two batches if necessary). Remove skillet from heat and cover to keep warm.
Heat broiler or grill. Remove shrimp from marinade and thread onto skewers. Broil or grill the shrimp until just done, about 5 to 6 minutes. Serve the shrimp with the spinach.