As with all Mexican foods there are regional variations. In Oaxaca the rajas are made with charred chile de agua chiles, lime, onion, salt and oregano then served with meats, stews and enfrigoladas. In central Mexico they use the chile manzano, or the peron chile. (Diana Kennedy, Art of Mexican Cooking).
rajas being sautéed with onions and seasonings
In addition to being used as a side dish, rajas are even used in tamales and tacos. This simple dish can be made ahead of time (the dish improves after it sits awhile).
You can use different peppers or seasonings but this is a good basic recipe, by Rick Bayless.
Chiles: Stem and seed the roasted chiles. You can remove the veins if you want a more mild rajas. Slice the chiles crosswise into 1/4" strips.
Heat the oil in a heavy sauté pan over medium heat and fry the onions until they brown, about 7 -8 minutes. Stir in the garlic and chile strips, cook 2 minutes.
Add the cream or broth, herbs, and bay leaves and simmer until the liquid reduces enough to just coat the vegetables. Remove the bay leaves, taste and adjust salt as needed.