Finely grate the zest of one of the limes and add it to the bowl of a mini food processor. Squeeze in the juice of the lime and add the garlic, cilantro, coarsely chopped scallions, cumin, and 1 teaspoon salt and pulse until roughly chopped. Add 2 tablespoons of the olive oil and 1 tablespoon water and continue to pulse until the mixture becomes a chunky sauce.
Toss the halibut in a bowl with all but 1 tablespoon of the sauce and let marinate for 10 to 15 minutes while you prepare the cabbage and the sour cream.
In a large bowl, toss the remaining 1 tablespoon cilantro sauce with the cabbage, finely chopped scallions, the juice of the second lime, 1 tablespoon of the olive oil and 1/2 teaspoon kosher salt. In a small bowl, stir together the sour cream, the juice of the remaining lime and a pinch of salt.
Heat a large nonstick skillet over medium-high heat. Add half of the remaining 1 tablespoon olive oil. Add about half of the fish, so the pieces are in a single layer, and cook, turning, until the fish is just opaque and flakes easily with a fork, about 4 minutes. Transfer the fish to a plate and cook the remaining fish with the remaining oil. Divide the fish among the tortillas and top with cabbage. Serve with the sour cream and salsa verde on the side.