Using a mandolin or sharp chef’s knife slice radishes into paper thin rounds and cabbage into thin strips. Transfer to a medium bowl and mix in the scallions, 1 Tbsp. olive oil, lime juice and 1/4 tsp. of salt. Massage and squeeze the cabbage with your hands to help it release its liquid and begin wilting. Set aside.
Pat the cod dry with paper towels; season with salt, pepper and a pinch of cumin on both sides. In a large pan, heat 2 Tbsp. olive oil on medium-high heat. Add the cod and cook until golden brown and cooked through, about 3 or 4 minutes per side.Set a large pan over medium-low heat. Arrange 2-3 tortillas in a single layer. Flip them after 30 seconds or so. As they come off the stove, wrap them in a clean tea towel. Repeat with a second batch. Divide cod between 4 tacos. Top with cabbage slaw and cilantro leaves.