Put the cauliflower steaks in a large resealable bag or dish. Dissolve salt in 5 cups of water and pour over the cauliflower. Set aside for 45 minutes.
Simmer the vinegar, sugar, and fennel seeds in a small saucepan over medium heat for 5 minutes. Add the tomatoes and cook, stirring, until soft, about 2 minutes. Set aside.
Preheat an outdoor grill or a grill pan over medium heat. Drain the cauliflower and pat dry. Brush with olive oil and grill, covered, until lightly charred and crisp-tender, about 6 minutes per side.
Serve with tomato relish over steaks.