Step 1Heat the olive oil in a wok over medium-high heat. Add the mustard seeds and cook until they pop, about 30 seconds.
Step 2Add the green onions and garlic and stir for a few seconds. Add the mustard, lemon juice, salt and cayenne and stir until smooth, then stir in 2 tablespoons water.
Step 3Add the broccoli florets. Cover and cook 3 to 4 minutes, then add the shrimp. Cover and cook 4 to 5 minutes, until the broccoli is crisp-tender and the shrimp are just done.