For the quick pickled onions: Combine the vinegar, sugar, salt, peppercorns, bay leaf and 1/4 cup water in a small saucepan. Bring to a boil and stir to dissolve the sugar and salt. Put the jalapeno and red onion in a medium heatsafe bowl and pour the hot vinegar mixture over. Let cool to room temperature. (The onions can be made ahead and refrigerated for up to a week.)
For the taco cups: Lightly spray the tortillas on both sides with cooking spray and dust with the chili powder. Fit each tortilla into a cup of a standard muffin pan, pleating the edges and making a space in the center to form a cup. Stagger the tortillas in the muffin pan so they are not bunched together. Bake until crisp and browned on the edges but still pliable in the center, about 8 minutes.
For the iceberg slaw. Whisk together the mayonnaise, buttermilk, mustard, garlic powder and lime juice in a large bowl. Season lightly with salt and pepper. Add the lettuce, carrot, cilantro, scallion and bell pepper and toss to coat the vegetables in the dressing.
For the taco filling: When ready to serve, heat a grill pan over medium heat. Grill the sausage slices, turning frequently, until heated through, 3 to 4 minutes.
To serve, divide the sausage among the taco cups. Top with some slaw, pickled onions, Cheddar and avocado. Serve with lime wedges.