Preheat the oven to 450°F. Lightly slick a rimmed baking sheet or large roasting pan with olive oil.
Tear the bread into roughly 1-inch chunks, toss in a food processor, and grind until fine crumbs form. You should have about 1/2 cup bread crumbs. Transfer to a small bowl.
Toss the almonds coarsely in the processor and process until finely ground. Add to the bowl with the bread crumbs. Mix in the scallions, lemon zest (you should have about 4 loosely packed teaspoons), and 1 tablespoon oil. Season to taste with salt and pepper.
Place the fish on the baking sheet, arranging it with the slightly curved side of the fillet facing up. Brush the fish with olive oil and squeeze the juice from the lemon halves over the fish. Season with salt and pepper, then spread the mustard lightly over the fillets.
Just before baking, divide the bread crumb mixture among the fillets, pressing the mixture onto the top of each fillet. Bake until the fish is springy to the touch and the bread crumbs begin to brown, about 10 minutes per inch of thickness. Transfer baked fish to plates and serve right away.
