Bring a large pot of water to a boil. Add the soba noddles and cook according to package directions. Drain and rinse with cold water.
Whisk the soy sauce, sesame oil and sugar together in a bowl and pour the mixture over the cooked noodles in a large bowl. Add the scallions and toss to combine.
To make the broccoli, mix the oil and garlic together in a large bowl. Add the broccoli spears and toss to coat with the oil mixture.
Transfer the broccoli to a baking sheet, spreading it out in a single layer. Roast in the oven, turning once, until the broccoli starts to brown, about 12 minutes. Sprinkle the sesame seeds on top and roast another 5 minutes until the broccoli is cooked and sesame seeds are toasted.
Remove the pan from the oven and place the broccoli in a bowl. Drizzle the soy sauce and vinegar on top and toss to combine. Season with salt and pepper to taste.
To serve, divide the noodles between four plates and arrange the tuna slices on top. Serve with sesame roasted broccoli. Garnish with scallions and sesame seeds.
One serving (broccoli): Calories 109; Fat 7.3g (Sat 1.1g); Protein 3.7g; Carb 8.4g; Sugars 2g; Fiber 3.2g; Sodium 125mg