Combine soy sauce, brown sugar, rice vinegar, ginger, and garlic in a saucepan. Simmer 2 minutes, whisk in cornstarch slurry, simmer 1 more minute until thickened.
Heat neutral oil in a cast-iron skillet over high heat until shimmering.
Season radicchio wedges with salt. Sear cut-side down for 3-4 minutes until dark and charred, then flip and sear the other side for 2-3 minutes.
Check that outer leaves are charred while the core stays firm.
Remove from heat and drizzle the warm teriyaki dressing over the charred wedges.
Transfer to a platter and finish with sesame seeds and sliced green onion.