Heat corn tortillas on a comal over medium heat until pliable. Transfer to a tortilla warmer to keep warm.
Cut parchment paper (½ sheet pan size) into quarters, large enough to enclose the tacos.
Add the filling to each tortilla and fold to form a taco. Place on a piece of parchment, brush both sides with chili oil, top with sliced onion, and fold the paper to enclose. Repeat with the remaining ingredients.
Line the bottom of the steamer basket with aluminum foil. Arrange tacos neatly inside and cover with parchment.
Fill the bottom of a steamer pot with water, stopping two inches below the steamer basket. Bring to a boil over medium-high heat, then reduce to a simmer.
Place the steamer basket over the water, cover, and steam for 15 minutes. The tacos are ready once they are heated through and soft.