These Japanese rice triangles can be made simply with seasoned rice or filled with vegetables or protein. Cooking a bit of starchier short-grain white rice with the brown ensures they hold together—but still gives you a filling dose of fiber.
Combine water, brown rice, white rice and salt in a medium saucepan. Cover and bring to a boil over high heat. Reduce heat to low and simmer gently until the water is absorbed and the rice is very tender, about 40 minutes. Remove from heat and let stand, covered, for 10 minutes. Stir in vinegar and spread the rice on a large baking sheet until just cool enough to handle, about 5 minutes.
Meanwhile, combine salmon, scallion and mayonnaise in a small bowl. Place seaweed strips on your work surface. Spread sesame seeds on a plate.
Line a small (4-ounce) ramekin with plastic wrap, leaving about 4 inches hanging over the sides. Scoop 2 tablespoons of the rice into the ramekin and top with 1 teaspoon of the salmon mixture in the center. Top with another heaping tablespoon rice and spread to cover the salmon. Bring the edges of the plastic up and twist the ends together, compacting the rice into a ball.
Untwist the plastic to loosen it a bit, and form the rice ball into a triangle shape about 3/4 inch thick. Unwrap and place the base of the triangle in the center of a piece of seaweed. Bring the sides of the seaweed up and press gently with a damp finger to adhere to the rice. Dip the edges of the triangle in sesame seeds. Repeat with the remaining ingredients to make 16 onigiri.
Refrigerate for up to 2 days.