In a small bowl, toss the beef with the cornstarch to coat.
Heat a generous drizzle of canola oil and a small drizzle of sesame oil in a large skillet set over medium-high heat and sauté the broccolini for 2-3 minutes, until bright green and tender-crisp.
Transfer to a plate, add more oil and cook the beef for 2-3 minutes, until just cooked.
Add the soy sauce, vinegar, brown sugar, garlic, ginger and about 1/4 cup water and bring to a simmer, stirring until slightly thickened.
Return the broccolini to the pan and toss to combine.
Serve with rice, topped with green onions and sesame seeds.