1 teaspoon neutral oil (such as grapeseed or canola)
2 scallions, white and pale green parts
2 cups coarsely chopped stemmed kale
1 rib celery, coarsely chopped
1/4 cup crumbled feta cheese
1–2 tablespoons extra-virgin olive oil
Kosher salt and freshly ground black pepper
Preheat the oven to 450ºF. Using the neutral oil, lightly grease a rectangular baking dish large enough to accommodate the fillets in a single layer.
Place the halibut fillets in the prepared baking dish and season lightly with salt and pepper. Use a sharp knife to slash several deep cuts in the top of each fillet, being careful not to cut all the way through the fish.
In a food processor, combine the scallions, kale, celery, and cheese and process until the ingredients are crumb-sized. Add 1 tablespoon of the olive oil and pulse until a paste forms, adding more oil as needed to achieve a good consistency. Season with salt and pepper.
Stuff the pesto into the slits in each fillet, filling them with as much pesto as will fit and allowing it to overflow.
Roast until the fish is opaque at the center when tested with the tip of a knife, about 15 minutes. Let cool for 3 minutes before serving.