Slice the tofu crosswise into 4 one-inch slabs. Then cut each slab into two triangles (total of 8 pieces). Blot with paper towels.
Spray a large skillet with cooking spray. Heat pan on medium-high heat and add oil. When oil is hot, add tofu and fry over medium-high heat until golden. Turn and cook on the second side.
While the tofu is cooking, add soy sauce, peanut butter, vinegar, brown sugar, pepper flakes, water and half the onions to a food processor or blender. Process until smooth.
When tofu is lightly browned, add sauce to pan and cook until bubbling. Turn off the heat, scatter toasted sesame seeds and remaining green onions over the top and serve.