CHAR BLOOD ORANGES (STAGE 1): Heat dry cast-iron skillet over medium-high. Place 2 orange halves cut-side down; char 12-15 minutes until mostly black. Cool, juice, and strain seeds. Should taste like roasted marmalade.
BUILD AROMATIC FENNEL BASE (STAGE 2): In Dutch oven, heat 2 tablespoons olive oil over medium. Add fennel wedges cut-side down; cook 5-6 minutes until golden, flip 3-4 minutes. Remove. Add 1 tablespoon oil, sliced fennel, 1 sliced onion, pinch salt. Cook 8-10 minutes. Stir in lemon-family zest and 2 teaspoons crushed fennel seeds; cook 30 seconds. Add 3 cloves garlic; cook 30 seconds.
BUILD TOMATO-CITRUS BASE: Stir in 2 tablespoons tomato paste; cook 2 minutes. Add 1 cup wine; simmer hard 3-4 minutes until reduced by half. Add crushed tomatoes, 2 1/2 cups stock, charred orange juice, bay leaf, bloomed saffron, salt, pepper. Simmer 10 minutes.
ADD COMPLEXITY: Return caramelized fennel wedges. Add 2/3 cup olives (and capers if using). Simmer 5 minutes. Taste and adjust.
STAGE SEAFOOD: If using shellfish, add first, cover, steam 4-5 minutes. Reduce to barest simmer. Nestle 1 1/4 pounds fish, cover, cook 4-5 minutes. Add 12 ounces shrimp/scallops, cover, cook 2-3 minutes.
FINISH WITH YUZU (STAGE 3): Remove from heat. Let stand 1 minute. Whisk in 2 tablespoons cold butter. Stir in 2-3 tablespoons yuzu juice 1 tablespoon at a time, tasting. Stop when broth smells brighter. Adjust salt.
SERVE: Spoon into bowls. Top with 1-2 teaspoons gremolata and pinch flaky salt.