To make Sauce: In saucepan over medium heat, whisk soy sauce into peanut butter. Whisk in chili sauce, sugar and 1 1/2 cups water; add green onions and bring to a boil, stirring constantly. Reduce heat and simmer 1 minute. Remove from heat and cool completely. Will keep, covered, one week in the refrigerator.
Slice each 16-ounce block tofu crosswise into 6 (1/2-inch-thick) pieces; slice each piece diagonally into 2 triangles. Drain on paper towels for about 10 minutes.
For each serving, to order: In nonstick pan, heat oil over high heat; brown 3 triangles tofu on both sides. Remove from heat. Place tofu triangles on plate, slightly overlapping; pour 1/4 cup sauce evenly over tofu. Sprinkle with 1 teaspoon green onions and 1/2 teaspoon sesame seed.
Tip: Toss cooked fettuccine with enough sauce to lightly coat, or serve sauce with grilled pork, chicken or eggplant, or as a dip for raw vegetables.