Toss cauliflower florets in 2 Tbsp vegetable oil.
Season cauliflower florets with salt and roast in the oven at 400F until tender and golden brown.
Remove from oven and allow to cool.
Combine the white granulated sugar, red wine vinegar, and water and heat in a pot on the stove until the sugar is dissolved.
Stir in the dried oregano and red pepper flakes and remove from heat.
Pour liquid mixture in a bowl over the sliced jalapenos, sliced onion, diced mango, and roasted cauliflower and toss mixture to coat exterior of vegetables.
Season mixture with salt to taste.
Allow the pickled mango and vegetable mixture to sit for 30 minutes while you preheat the grill and cook the tuna fillets.
Preheat grill to 550F.
Clean grill grates well, to reduce the likelihood of the tuna sticking to the grill.
Season both sides of the tuna fillets with salt and freshly-ground black pepper before grilling.
Use the remaining vegetable oil and a piece of cloth to gently oil the surface on the grill where you will be cooking the tuna.
Cook tuna on both sides and remove from grill when cooked to rare or medium rare.
Allow tuna to rest for 2 minutes.
Cutting against the grain of the flesh, use a sharp knife to carefully slice the grilled tuna into ¼” slices.
Serve topped with pickled mango and pickled vegetables.