For the fish: Combine the ginger, cumin, coriander, turmeric, paprika, cinnamon, and cayenne pepper in a small bowl and mix to blend. Place the fish on a plate and brush the flesh side (both sides of the mahi mahi) with olive oil, and then sprinkle with salt and pepper. Sprinkle the spices very generously over the flesh side of the fish (both sides of the mahi mahi), using all of the spice mixture. Let marinate at room temperature while you prepare the sauce. Or, cover and refrigerate for up to 1 hour.
Meanwhile, prepare the sauce: Combine the yogurt, dill, 2 tablespoons olive oil, lemon juice and zest in a small bowl. Mix well. Season to taste with salt and pepper.
Heat a medium size nonstick skillet over medium heat. Add olive oil to coat. For salmon or steelhead: place the fish in the skillet skin side down and cook until the skin browns and crisps, about 7 minutes; turn over and cook until the fish feels springy to the touch, about 2 minutes longer. For the mahi mahi: add the fish to the skillet and cook until springy to the touch about 4½ minutes on each side.
Transfer the fish to a work surface and cut lengthwise in half. Divide the arugula between 2 plates. Arrange the fish atop the arugula. Spoon the sauce over and serve.