How to Make It
Heat canola oil in a medium saucepan over medium. Add onion, fennel, and garlic to pan; cook, stirring often, until onion is translucent, about 8 minutes. Stir in tomatoes, chili powder, and paprika. Reduce heat to medium-low, and simmer until tomatoes are softened and slightly broken down, 5 to 6 minutes. Stir in stock, and increase heat to medium; cook until reduced to about 3 cups, about 10 minutes. Season with 1 teaspoon salt. Remove sauce from heat, and cover to keep warm.
Rinse fish under cool running water, and pat dry with a paper towel. Place on a large rimmed baking sheet; drizzle both sides with grapeseed oil, and sprinkle all over with remaining 4 teaspoons salt.
Open bottom and top vents of a charcoal grill completely. Light 2 charcoal chimney starters filled with briquettes (about 81/2 cups of briquettes per chimney). When briquettes are covered with gray ash, pour in an even layer on bottom grate of grill. (Internal temperature should be 375°F to 400°F).
Place fish on oiled grate, and grill, uncovered, until charred and meat pulls away from bones, about 9 minutes for large fish and 5 minutes for smaller fish. Using 2 large fish spatulas, carefully flip fish, and grill, uncovered, until fish is opaque and flaky, 6 to 8 minutes.
Using fish spatulas, carefully transfer fish to a serving platter; spoon sauce over fish, and serve immediately.