Preheat the oven to 325° F. Line a baking sheet with parchment paper or a silicone baking mat.
In a large bowl, combine the Parmesan, fontina, and Asiago cheeses until evenly mixed. In a separate bowl, stir together all the seeds, dried garlic, and dried onion.
Sprinkle the cheese mixture evenly onto the prepared baking sheet, forming a 12×8-inch rectangle with an even thickness. Sprinkle the seed mixture evenly over the cheese, then finish with flaky sea salt.
Bake for 35–40 minutes, rotating the pan halfway through, until the cheese has melted and turned a light golden brown.
Remove from the oven and allow to cool completely on the pan. Once fully cooled and crisp, break into approximately 1-inch cracker pieces.
Notes
Makes about 8 servings of 10 to 12 crackers each.If you prefer a less salty snack, leave the salt off, or cut it in half.Store crackers in an airtight container at room temperature for up to 7 days.For longer storage, refrigerate for up to 2 weeks or freeze for up to 2 months.