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1
Preheat oven to 400u00b0.
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2
Make rub: Toast fennel seeds in a small dry skillet over medium heat until fragrant, about 1 minute. Crack peppercorns in a mortar and pestle. Add fennel seeds and salt and crush together until coarsely ground. Stir in orange zest and oil. Cover tuna all over with rub, then chill.
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3
Meanwhile, make ratatouille: Heat 1/4 cup oil in large roasting pan over medium-high heat. Add eggplant and cook, stirring occasionally, until tender, 10 to 15 minutes. Transfer eggplant to a medium bowl. Return pan to medium-high heat and add 1 tbsp. oil. Add sliced fennel and cook, stirring occasionally, until tender and browned, 8 to 10 minutes; transfer fennel to bowl of eggplant. Return pan to medium-high heat and add 1 tbsp. oil. Add squash and onion and cook, stirring occasionally, until softened, 5 to 10 minutes. Add tomatoes, cook 1 minute, and return eggplant and fennel to pan. Add wine and parsley and cook, stirring, 2 minutes. Season to taste with salt and pepper. Push vegetables apart a little to make space for tuna.
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4
Heat remaining 2 tbsp. oil in a medium nonstick frying pan over medium-high heat until shimmering. Sear tuna until browned, 30 seconds to 1 minute on each side. Nestle tuna into vegetables and roast 6 to 8 minutes, or until opaque on the outside but still pink in most of center. Let tuna rest 10 minutes, uncovered (it will cook all the way through). Stir fennel fronds into vegetables.
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5
Carve tuna into 1-in.-wide slices and serve with vegetables and a drizzle of pan juices.
