For the dipper:
2 tablespoons olive oil
1 onion, finely chopped
1 large jalapeño chile, or 1 small poblano chile (seeded and finely chopped)
2 tablespoons chopped garlic (from about 6 cloves)
Salt and pepper
2 cups chicken stock
One 14.5-ounce package fire-roasted tomatillos (I like Frontera brand), or one 14-ounce can whole tomatillos (drained)
½ cup sour cream, Mexican crema, or crème fraîche
1 tablespoon fresh lime juice (from 1 lime)
For the sandwiches:
1 small red onion, very thinly sliced with a sharp knife or on a mandoline
1 lime, juiced (about 2 tablespoons)
Salt and pepper
4 tablespoons butter, at room temperature (or ¼ cup mayonnaise)
8 slices sourdough bread or good-quality white bread (¼- to ½-inch-thick each)
Pickled jalapeño slices
Fresh cilantro or flat-leaf parsley leaves
8 slices pepper jack
1 pound sliced roasted turkey breast
8 slices Swiss cheese
For the dipper, heat a large saucepan over medium-high. Add the oil, two turns of the pan. Add the onion, chile, and garlic; season with salt and pepper. Cook, stirring occasionally, until softened, about 5 minutes. Add the stock and tomatillos. Bring to a boil. Simmer until the tomatillos break down, 5 to 10 minutes. Stir in the sour cream. Remove from heat and transfer to a blender. Add the lime juice. Puree to make a sauce. Return the dipping sauce to the pan and keep warm over low heat.
For the sandwiches, heat a griddle pan over medium to medium-high.
In a small bowl, toss the onion with the lime juice; season with salt and pepper. Spread the butter on 1 side of each slice of bread. Build the sandwiches with the bread (buttered sides out), the pickled jalapeños, cilantro, pepper Jack, turkey, Swiss cheese, and pickled onion. Griddle the sandwiches, turning occasionally, until the bread is deep golden and the cheeses melt, about 5 minutes.
Cut the sandwiches from corner to corner. Serve with the dipper in ramekins or small cups.