Cook your rice before starting the vegetables. Set aside.
In a small mixing bowl, whisk together the soy sauce, mirin, and sesame oil. Set aside.
In a wok or large heavy-bottomed skillet over medium-high heat, warm the vegetable oil. Once hot, add the onion slices, garlic, and ginger. Sauté the aromatics until they just start to soften, 1 to 2 minutes.
Add the broccoli and mushrooms. Cook, stirring often, for about 2 minutes until the vegetables start to wilt.
Add the corn and shrimp and sauté for 2 minutes, stirring often, or until the shrimp just start to turn pink.
Turn the heat to high and add the soy sauce mixture. Toss quickly to coat. As soon as the wet ingredients start to boil, turn off the heat.
Place the rice in individual serving bowls. Spoon the shrimp and vegetables over the rice. Garnish with sesame seeds and scallions.
Serve immediately with the yum yum sauce on the side.