With locatelli pecorino romano.
My wife and I had the same issue when we started out. We’ve solved it two different ways.
Option 1: Use 3 parts pecorino romano, 1 part parmesano, mix in a bowl and then slowly add to pasta while vigorously stirring (pasta should be very hot still to melt cheese)
Option 2 (my personal preference): Combine a bit of cacio y pepe recipe with a bit of carbonara recipe :)
Mix your cheese with egg (we use 3 eggs + 1 yolk, 3oz pecorino romano, 1oz parm) until it’s a slurry. Add pepper to the mixture. Then add to pasta and stir vigorously.
You’re missing the main ingredient to make it creamy: cooking water.
You need to mix the cheese with one ladelful of water (picked when the pasta is almost ready so there’s some gluten in it).
Then drain the pasta and stir it in the sauce.
If you don’t do that there’s pretty much no way to avoid clumps.
