Add cauliflower florets to a baking dish or tray and combine with 1.5 tbsp olive oil, 1/4 tsp salt & 1/8 tsp black pepper. Roast in the oven at 200C/400F for around 20mins, or until lightly charred and fork tender (toss once throughout). Timings will depend on the size of the florets, so just be vigilant. You don't need them completely soft as they'll carry on cooking in the pasta bake.
With around 8-10mins to go, add the pasta to salted boiling water and cook until al dente. You want it still a touch hard, otherwise, it'll end up too soft once baked. Drain when needed.
Meanwhile, melt 4 tbsp butter in a large pan over medium heat and stir in 4 tbsp flour. Gradually add in 1 cup of veg stock then 2 cups of milk, whisking as you go to avoid lumps forming. Once the sauce is thick and bubbly, turn off the heat and stir through 1 cup cheddar and 1 cup gruyere. Stir in 1 heaped tsp dijon alongside 1/4 tsp garlic powder, onion powder, white pepper, salt and 2 pinches nutmeg.
Stir in the drained pasta alongside the roasted cauliflower. Pour into a baking dish then roughly cover with more cheddar. Place the dish on a baking tray and pop in the oven at the same temp for 20-25mins, or until deep golden and bubbly. At this point, you can cook the bacon, either in the oven or on the stove.
Dice the bacon, scatter over the pasta bake then tuck in and enjoy!