Step 1

Preheat oven to 375°. Heat oil in a medium skillet over medium. Cook onion, stirring occasionally, until golden brown and softened, 8–10 minutes; season with salt. Let cool.

Step 2

Pulse cream cheese and mayonnaise in a food processor until smooth.

Step 3

Toss cornstarch, ¾ cup cheddar, and ¼ cup Monterey Jack in a medium bowl to coat cheese. Mix in cream cheese mixture and cooked onion; season with salt and pepper. Scrape into a 1-qt. baking dish and top with remaining cheddar and Monterey Jack. Bake dip until golden and bubbling, 20–25 minutes. Let dip cool 5 minutes, then top with chives and peperoncini. Serve with crackers.