Toast the coriander and cumin seeds in a small pan over medium heat until fragrant, about 2 minutes. Put the seeds in a blender along with the olive oil, cilantro, parsley, mint, paprika, red pepper flakes, garlic, lemon zest and juice and 1 teaspoon salt. Blend until smooth. Remove half of the chermoula to a bowl and set aside.
Sprinkle the steak with salt and pepper and place in a resealable plastic bag. Pour in the remaining half of the chermoula. Seal the bag and marinate at room temperature for 1 hour.
Preheat a grill to medium high.
Grill the steak, turning once, until charred and cooked to medium rare, about 3 minutes per side. Transfer to a cutting board and let rest for 5 minutes. Slice the steak across the grain. Serve with the reserved chermoula.