Toss the fennel, onion, and crushed red pepper flakes together and pack them into a two-cup jar or sealable container. Add the vinegar, water, sugar, and salt to a small saucepan, bring to a simmer over medium-low heat, and cook until the sugar and salt dissolve. Alternatively, you can heat this mixture in the microwave for 30 to 45 seconds then stir until the sugar and salt dissolve.
Pour the hot liquid over the fennel and onion mixture so that it is fully submerged then set aside to cool to room temperature. Seal the jar or container and transfer to the fridge for at least 30 minutes to lightly pickle or for up to two weeks.
Combine the mayo, Parmigiano-Reggiano, capers, parsley, and lemon zest in a small bowl. Season with pepper.
Split the buns in half and toast or grill if desired. Spread one side of each bun with the mayo mixture and the other side with mustard.
Top each with three or four slices of the Porchetta-ish roast as well as some of the pickled fennel and onion and a scattering of hot peppers, if desired. Enjoy!