Grate 12 tablespoons butter on a large hole grater. Slice 6 ounces feta into ⅛-inch thick planks.
In a mixing bowl, combine 2 cups (250g) all-purpose flour, ½ cup (60g) rye flour, 1 tablespoon baking powder, 1 ½ teaspoons cumin seeds, 2 teaspoons dried onion flakes, 1 teaspoon salt and 1 teaspoon sugar.
Add shredded butter to flour mixture and lightly mix with fingers until combined and butter pieces are completely coated in flour. Add feta to dough and toss to combine; feta should break apart into large shards.
Form a well in center of flour butter-mixture; pour 1 cup (230g) buttermilk into well. Using a spoon, mix dough gently until a dough comes together.
Transfer dough to a work surface and lightly press dough into a rough 8✕5-inch rectangle (about 1-inch thick)
Fold rectangle into thirds (like a letter) and press down into a 1-inch thick rectangle. Repeat folding and pressing twice more.
Shape rectangle into a rough 8✕5-inch rectangle (about 1¼-inches thick). Transfer to a parchment lined baking sheet and chill 30 minutes.
Meanwhile, heat oven to 450ºF (232ºC) with rack set in middle position.
Melt remaining 4 tablespoons butter in a saucepan over medium heat. Add remaining 1 teaspoon cumin seeds and 1 teaspoons onion flakes; cook just until fragrant, butter is lightly browned and spices are toasted, 2 minutes. Remove from heat.
Cut biscuit dough in half lengthwise then cut each half into 4 squares.
Brush biscuits with spiced butter. Sprinkle biscuits with flaky sea salt and bake until risen and golden brown on tops and sides; 27–29 minutes, rotating halfway through.
If you have leftover spice butter, serve it with biscuits.