Toast 1/2 cup of the couscous in a dry skillet over medium heat. Bring the vegetable stock to a boil, stir in the toasted and untoasted couscous and some salt, cover, and simmer until the couscous is cooked and the stock absorbed, about 8 minutes. Remove from the heat, fluff with a fork, and set aside.
Heat the grill to medium-high. Brush the eggplant, zucchini, yellow squash and red onion with canola oil, season with salt and pepper and grill until both sides are charred and the vegetables tender. Remove, cool slightly, and cut into large chunks.
Toss the tomatoes with canola oil, season with salt and pepper and grill in the grill basket until they are just lightly charred.
Brush the tuna steaks with canola oil, season with salt and pepper and grill until golden-brown and medium-rare, about 2 minutes per side.
Put the chopped vegetables in a large bowl. Add the couscous, basil, pine nuts, garlic, Parmesan, 1/4 cup olive oil, lemon zest and juice. Season with salt and pepper and gently toss.
Slice the tuna into thick slices. Mound the couscous on a platter, arrange the tuna on top and drizzle with additional olive oil. Garnish with whole basil leaves.
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