How to Make It
Heat a medium skillet until hot to the touch. Add the tomatillos and cook over high heat, turning, until blackened in spots but not mushy, about 5 minutes. Transfer the tomatillos to a plate. Add the oil to the skillet along with the cumin, coriander and cayenne and cook over moderate heat just until fragrant, about 30 seconds; let cool.
Coarsely chop the tomatillos and transfer to a medium bowl. Add the scallions, garlic, jalapeño, cilantro, lime juice and spiced oil. Season the salsa with salt and pepper.
In a bowl, stir the sour cream and mayonnaise with 1/2 cup of the salsa.
Lightly toast the bread. Spread the creamy salsa on each slice of toast and top half of the slices with the arugula, avocados and Jack cheese. Close the sandwiches, cut them in half and serve with the remaining salsa on the side.