Press the tofu for 10 minutes while preparing the zucchini and starting the water for the pasta. See note below.
Start boiling the water for pasta in a large pot while preparing the tofu and vegetables.
Remove the tofu from between the pressing plates, discarding the liquid. Cut tofu in half lengthwise down the middle so you have 2 large pieces. Sprinkle both sides of each tofu piece with salt.
Heat 2 tablespoons of the olive oil in a large deep skillet over medium-high heat. Fry the 2 large pieces of tofu until golden on both sides (about 3 to 4 minutes each side). Remove tofu from skillet. Allow to cool. Dice tofu into bite-size pieces and set aside.
Pour the remaining olive oil into the same skillet over medium heat. Add garlic and zucchini. Sauté for a few minutes. Add salt, black pepper, chili flakes, and sugar. Continue to cook until zucchini is soft. Add diced tofu to the skillet and continue to cook over low heat while preparing pasta, checking and stirring periodically. Do not overcook.
Cook pasta per package instructions until al dente. When the pasta is ready, remove with tongs and toss with zucchini mixture. Mix in watercress and salt to taste.
Note: To press tofu, open the package over the sink, as the liquid in the package can spurt out when opening. Press the tofu between two plates and set in the sink. Put a heavy weight on top of the upper plate. A large jar filled with food or water will work. When ready to use, a lot of the liquid will have drained from the tofu. Discard this liquid.