Make shrimp aioli: Gently boil 2 cups shrimp or fish stock until reduced to 2 tablespoons and slightly thickened. Remove from pan and cool. Once cool, add to a food processor with egg yolk, garlic, lemon juice, and vinegar. With motor running, add olive oil and process until emulsified. Salt to taste. Refrigerate.

Cut baguette diagonally into ¼-inch slices. Toast lightly; set aside.

Shave fennel on a mandolin about 1/ 8 inch thick. Toss with a little olive oil and reserve in refrigerator until serving time.

On a baking sheet, arrange tuna, shrimp, and calamari, and season with salt and pepper. Coat a large, deep sauté pan over high heat with olive oil. Pan-sear tuna cubes 5 seconds on all sides; remove from pan to a paper-towel-lined tray. In same pan, sear shrimp and calamari for 1 minute; remove to tray with tuna. In same pan, add chorizo and stir to crumble. Remove. Add cod and bass or halibut; pan-sear. Deglaze pan with 3 cups shrimp or fish stock, and then simmer until cod and bass are tender and done, about 3 minutes. Return seared tuna, shrimp, calamari, and chorizo back into pan and simmer for 2–3 minutes. Add berbere and salt to taste.

Divide stew into bowls and sprinkle with fennel, parsley, and more berbere spice if desired. Place one piece of bread and 1 tablespoon shrimp aioli on top. Serve immediately.