Preheat your oven to 375°F (190°C). Place a small (6–8 inch) cast iron skillet on a sheet pan to catch any bubbling overflow and make it easier to move in and out of the oven.
Finely chop the sweet onion. You want small, even pieces so they soften and almost melt into the dip as it bakes.
In a medium mixing bowl, combine the chopped onion, shredded sharp cheddar cheese, and the entire can of condensed soup (do not dilute with water or milk). Stir until everything is thoroughly mixed and no pockets of dry cheese remain.
Transfer the mixture into the small cast iron skillet, smoothing the top into an even layer. Press it out to the edges so it browns evenly.
Bake, uncovered, for 25–35 minutes, or until the dip is bubbling vigorously around the edges and the top is deeply golden brown with caramelized spots. The cheese should be fully melted and the onions tender.
Carefully remove the skillet from the oven and let the dip rest for about 5–10 minutes. This short rest helps it thicken slightly so it clings better to chips and bread while still staying wonderfully gooey.
Serve the dip warm, directly in the cast iron skillet, with your favorite dippers arranged around it. Remind curious guests—with a bit of pride—that it’s only three ingredients.
Variations & Tips
For a slightly different flavor profile while staying close to the spirit of the recipe, you can swap the type of condensed soup: cream of celery will give a lighter, herbaceous note, while cream of mushroom adds earthiness. If you want a sharper, more assertive flavor, use extra-sharp cheddar, or blend half cheddar and half Monterey Jack for a stretchier texture. You can also play with the onion: very thinly sliced onions instead of finely chopped will give you more distinct strands of sweet, soft onion in each bite. To adjust for different ovens, if the top isn’t browning by the time the dip is bubbling, move the skillet to the top rack for the last 3–5 minutes, watching closely so it doesn’t scorch. Leftovers reheat well in the same cast iron skillet at 350°F until warmed through and bubbly again; just cover loosely with foil if it’s browning too quickly on top.