For the biscuits: In a large bowl, combine the flour, sugar, baking powder, and salt and stir with a fork to mix.
Working over the bowl, use a paring knife to cut the 1⁄2 cup ice-cold butter into slivers, letting them drop into the flour mixture. Use your fingers to massage the butter into the flour mixture until it resembles coarse sand. Stir in the everything spice.
Make a well in the flour mixture. Pour in the buttermilk mixture and use a spatula to fold together until thoroughly combined. The dough will be sticky.
Without kneading, use your hands to form the dough into a ball. (It’s helpful to flour your hands to prevent them from sticking to the dough.) Transfer to a heavily floured, clean surface, like a cutting board or baking sheet. With the palms of your hands, flatten the dough into a rough 11 by 13-inch rectangle about 1 inch thick. Using a narrow-mouth mason jar, cup, or 21⁄2-to 3-inch cookie cutter, cut circles from the dough. (You can flour the mouth of your jar or cutter if it gets sticky.) Gather the dough scraps together, flatten the dough again, and cut out more circles. You should have about 24 biscuits.
Wrap each biscuit individually with plastic wrap and store in an airtight container or ziplock bag. Chill for up to 1 day or freeze for up to 6 months. If packing frozen biscuits in your cooler, they will begin to defrost and hold for up to 3 days, assuming the cooler is kept very cold.
Defrost the biscuits a day before baking. You want them to be very cold when you’re ready to cook; this is essential for getting that flaky texture.
In a 12-inch cast-iron skillet with a lid over medium heat, melt 1⁄2 tablespoon of the remaining butter, allowing it to spread evenly. Once the butter begins to foam (before it browns!), add 8 to 10 biscuits directly from the cooler.
Cover the skillet and cook until the biscuits are crispy and brown on the bottom, about 8 minutes. If some biscuits are cooking faster than others, use a spatula to shift them around in the pan—the biscuits in the middle will cook the fastest. Flip the biscuits and cook until the other side is crispy and brown, about 5 minutes more. Remove the pan from the heat and let the biscuits continue to bake in the pan, covered, until ready to serve. To test if the biscuits are fully baked, rip off a small piece and squeeze it; if it is a dough-like consistency, continue cooking the biscuits for 3 to 5 minutes. Repeat this process with the remaining biscuits, adding another 1⁄2 tablespoon butter before cooking.
Serve warm, directly from pan. Store leftovers in an airtight container at ambient temperature for up to 5 days.
For the everything spice: Line a plate with a paper towel.
In a small pan over medium-low heat, melt the 1 tablespoon butter. Once the butter begins to foam (before it browns!), add the onion and cook, stirring occasionally, until brown and crisp, 5 to 7 minutes. Transfer the onion to the prepared plate.
Add the 1 teaspoon butter to the pan and let melt. Once the butter begins to foam (before it browns!), add the garlic and cook, stirring frequently, for 3 to 5 minutes, watching carefully so that the garlic doesn’t burn. Transfer to the plate with the onion.
In a small bowl, combine the sesame seeds, caraway seeds, poppy seeds, and salt. Add the onion and garlic to the seed mixture, stir together with a fork, and set aside until cool.
Store in a lidded jar or airtight container in a cool, dark place for up to 3 days, or chill for up to 2 weeks.