Cut 1 bunch broccoli into florets; peel and slice the stems. Put in a steamer basket over boiling water; cover and cook 5 minutes. Transfer to a bowl and toss with 1 teaspoon sesame oil and 1 tablespoon sesame seeds. Whisk 1/2 cup mayonnaise, 1 tablespoon each Sriracha and lemon juice, and salt. Drizzle on top.