Set the jalapeños over a gas burner on medium-high heat or on a baking sheet under a broiler. Roast, turning occasionally until the skin chars all over, 6 to 7 minutes. Put the chiles in a paper bag or a bowl (cover with plastic) and let them steam.
Meanwhile, in a small bowl, whisk the mayonnaise, scallions, cilantro, lime juice and 2 Tbs. of the vegetable oil.
With a table knife, scrape the charred skin off the jalapeños. Discard the stems, cores, and seeds and chop the jalapeños finely. Stir about half of the jalapeños into the sauce, season to taste with salt, and add more jalapeños to taste—you may not want to use them all. Chill until ready to serve.
In a small bowl, combine the cumin, 1-1/2 tsp. salt, and the pepper. Rub the mixture onto both sides of the tuna steaks. Heat the remaining 2 Tbs. vegetable oil in a 12-inch heavy-duty nonstick skillet over medium-high heat. Add the tuna steaks and sear until browned on the first side, about 2 minutes. Flip and cook until browned on the second side, about 2 minutes more (the tuna will still be raw in the middle). Let the tuna rest for 2 minutes and with a very sharp knife, slice it crosswise into 1/4-inch-thick slices. Serve with the jalapeño sauce.