Heat the oven. Set the oven to 220°C (425°F) with the racks near the center. Start the rice or quinoa according to its package so it finishes about when the sprouts do.
Dry the sprouts. Rinse, then spread on towels and pat until no water beads remain. Trim the brown stem ends, remove yellow leaves and halve lengthwise.
Season. Toss the sprouts with the oil and salt in a bowl until lightly glossy.
Arrange. Divide between two rimmed sheet pans, turning each piece cut side down in a single layer with a little space between.
Roast. Roast for 20 to 25 minutes without stirring, checking from about 18 minutes. They are done when the cut sides are deep brown and the outer leaves look crisp, and a knife slides into the center with little resistance. Ovens vary, so go by the color.
Whisk the dressing. Shake or whisk the oil, lemon juice, mustard, garlic, capers, pepper and salt in a jar until it looks thick and cloudy.
Dress the tuna. Flake the drained tuna into a bowl and toss with about two thirds of the dressing and the red onion. Keep the remaining dressing for the bowls.
Cool the sprouts. Leave the roasted sprouts on the pans until they stop steaming.
Pack. Divide the grain among four containers, add the sprouts and the tuna in separate sections, and sprinkle with pumpkin seeds. Pack the remaining dressing in four small jars, then refrigerate.