Because this dish is ready just minutes after you sauté the tofu, have the watercress prepped and everything at your fingertips before you start to cook. The reduced orange juice in the glaze makes the flavor sing.
Lay tofu block on its side. Cut lengthwise into 4 equal slices then cut through block diagonally to make 8 triangles. Keeping block intact, place tofu on a plate. Place another plate on top and weight with a heavy can. Let stand 15 minutes.
Meanwhile, in small saucepan, bring orange juice to a boil over medium-high heat. Cook until reduced to 2 tablespoons, about 8 minutes. Transfer to small bowl and stir in shoyu, sake, scallion and 2 tablespoons water. Set aside.
Spread flour on a plate. Press each tofu triangle into flour, making sure both sides are well coated. Heat large, heavy skillet over high heat; add 1 1/2 tablespoons vegetable oil. Immediately add tofu and cook until lightly golden, turning once, 2 to 3 minutes per side. Divide tofu among 4 plates; set aside.
In same skillet, heat remaining 1 teaspoon vegetable oil over high heat. Add garlic and stir 10 seconds. Add orange juice mixture (be careful, it will steam quite a bit) and cook 30 seconds. Pour orange glaze over tofu triangles.
Add watercress to skillet and stir-fry over high heat until just wilted, about 30 seconds. Remove from heat. Stir in sesame seeds and drizzle with sesame oil; season with salt and freshly ground pepper to taste. Mound watercress alongside tofu. Serve hot.