1/2 cup vegetable oil
1 small white onion, finely chopped
1 clove garlic, chopped
1 jalapeno chile, thinly sliced crosswise
1 teaspoon ground cumin
1/2 pound tomatillos - husks removed; fruit rinsed and chopped
Salt and pepper
3/4 cup sour cream, plus more for serving
1 cup (packed) cilantro leaves
4 cups shredded roast chicken
10 small 6-inch corn tortillas
6 ounces (1 1/2 cups) shredded monterey jack
2 cups shredded romaine lettuce