In a medium bowl, stir together 2 tablespoons of the lime juice, 2 teaspoons of the red curry paste, 1 tablespoon of water, and a pinch of salt. Add the diced seitan and set aside in the refrigerator for at least 30 minutes.

While the seitan marinates, prepare the sauce. In a small bowl, whisk together remaining lime juice, tamari, peanut butter, remaining red curry paste, and brown sugar. Set aside.
Heat eh vegetable oil over medium-high heat in a large skillet. Add the seitan and marinade and sauté until marinade is absorbed and seitan begins to brown, about 7 minutes. Turn off the heat and stir in the bell pepper, carrot, scallions, cilantro and peanuts.

Arrange lettuce leaves on a platter. Divide the seitain filling between them, drizzle each one with sauce, and serve. To eat, fold or roll the lettuce leaf over the filling like a soft taco.