Preheat the oven to 400 degrees. Slice one of the lemons thinly, remove the seeds, and place the slices on a foil-lined baking sheet. Drizzle with 2 tablespoons olive oil. Roast 10-12 minutes or until slightly charred on the edges. Let cool completely.
Cut the feta into about ½” chunks.
In a jar or small bowl, make two layers of feta, red pepper flakes, fresh dill, celery leaves, peppercorns, and roasted lemon slices.
Squeeze the juice from half of the second lemon into the jar or bowl. Add enough olive oil to cover the contents. Chill for at least 8 hours before using.
The marinated feta keeps for 5-6 days in the fridge. When all the feta is used, the oil may be strained and used for salad dressings, etc.