Heat a large skillet over medium high heat and add half of the oil. Liberally season cod with salt, pepper, red pepper flakes, and cumin. Add fish to the pan and cook 4 minutes each side until fish is opaque and flakes easily with a fork, and set aside.
In a medium bowl, whisk together remaining oil, lemon juice and zest, and garlic. Add zucchini, avocado, and onion, stirring well to combine.
Serve fish atop arugula and top with the relish.