Using kitchen scissors, carefully open the tofu package. Drain the liquid. Place the tofu between 2 plates. Weight the top plate with a heavy jar for 20 minutes.
Prep the vegetables and start the water for the pasta.
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.