Preheat the oven to 450 degrees. Line a rimmed baking sheet with foil or parchment paper (for easy cleanup). Set aside.
Cut the leaves from the bottom of the cauliflower. Trim about ½” off the bottom of the stem.
Cut the cauliflower into ¾” slices. You should end up with at least 2 full “steaks” from the middle part of the head of the cauliflower. The other slices may fall apart into florets which is perfectly fine.
Lay the steaks onto the baking sheet and scatter the florets around, save them for another recipe, or for fresh-veggie snacking.
Drizzle the steaks and florets on all sides with the oil.
Combine the salt, pepper, ground mustard, paprika, garlic and onion powders in a small bowl.
Sprinkle the cauliflower steaks and florets evenly with the spice mixture on all sides.
Roast the cauliflower for 20 minutes. Remove from the oven and flip the steaks and florets. Roast for another 10 - 15 minutes, or until nicely browned and fork-tender.
Make the Gorgonzola Sauce while the cauliflower is roasting: In a shallow saucepan, melt the butter with the oil over medium heat.
Once melted, add the garlic and stir for one minute.
Add the heavy cream and ½ cup of the half and half to a measuring cup. Add the cornstarch and whisk together. Add to the saucepan and stir.
Bring to a gentle simmer over medium heat, stirring often. The mixture will thicken.
Once simmering, add the white pepper and Gorgonzola cheese and stir until melted. Taste the sauce before adding salt. The cheese probably has quite a bit of salt already. You may like it as is. Add any salt as desired.
Turn the heat down to low to keep warm while the cauliflower finishes cooking.
When the cauliflower has finished roasting, remove from the oven.
Serve the cauliflower steaks (and any florets) with the Gorgonzola Sauce drizzled over the top. (You may add about 1 to 2 tablespoons more of the remaining half and half to the cheese sauce if it has thickened more than you prefer.)
Garnish with the parsley and enjoy!