I offer you two ways to do it: (i) you all roll initiative, I average rolls (rounding up); (ii) one initiative roll is made with the average initiative bonus (rounding up).
The expected initiative roll for the PCs' block is the same with both methods naturally, but (i) has a higher variance while (ii) is more controlled.
I prefer (i) so you all still do
something when time comes to roll for initiative, but I expect (ii) is better for the PCs because it's not common for monsters to beat the PCs in initiative, which lower variance will enforce more strictly.