Heat sesame oil in a large skillet over medium-high heat. Add tuna; cook until browned, about 2 minutes per side. Remove skillet and slice into 1/2-inch cubes.

Place sesame seeds in a small skillet over medium-low heat; toast, tossing occasionally, until lightly browned, about 5 minutes. Remove from heat.

Combine tuna, sesame seeds, avocados, pomegranate seeds, scallions, ponzu sauce, cilantro, and Thai chile pepper in a large bowl.

Divide brown rice among 4 serving bowls. Top with tuna mixture.