Put the cauliflower slices in a large resealable plastic bag or baking dish. Dissolve 3 tablespoons kosher salt in 5 cups water and pour over the cauliflower. Set aside for 45 minutes.
Meanwhile, simmer the vinegar, sugar, fennel seeds and 1/4 teaspoon salt in a small saucepan over medium heat for 5 minutes. Add the tomatoes and cook, stirring, until soft, about 2 minutes more. Set aside.
Preheat an outdoor grill or grill pan over medium heat.
Drain the cauliflower and pat dry. Brush with extra-virgin olive oil and grill, covered, until lightly charred and crisp-tender, about 6 minutes per side. Serve the tomato relish over the steaks.
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