Heat the oven to 425°F. Place the poblano on a sheet pan and roast 12 to 15 minutes, turning once, until the skin blisters and chars. Transfer to a bowl, cover, and let it steam 10 minutes, then peel off the skin, remove the seeds, and dice.
Dice the onion and mince the garlic. In an oven-safe skillet over medium heat, cook the onion 4 minutes until soft, then add the garlic and diced poblano and cook 1 minute more.
Toss the of Oaxaca and of Monterey Jack together and spread evenly over the onion mixture in the skillet. Bake 10 to 12 minutes, until fully melted and bubbling at the edges. Finish under the broiler for 1 to 2 minutes if you want browned spots - watch it closely.
While the fundido bakes, make the elote. Grill or broil the of corn 8 to 10 minutes, turning every couple of minutes, until lightly charred in spots. A gas burner works too - hold each ear with tongs and char it directly.
Stir the mayonnaise, sour cream, and the juice of 1 lime together in a small bowl.
Brush the hot corn all over with the mayo mixture, then sprinkle with the of cotija, the chili powder, and chopped cilantro. Cut the remaining lime into wedges.
Serve the bubbling queso fundido right away with the of tortilla chips for scooping, alongside the elote and lime wedges. Everything goes on the table at once as one shared spread.
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