In a heated non-stick pan, add 2 teaspoons of oil and sauté the onion and bell pepper for about a minute. Dish out and set aside.
Using the same pan, add in the water with cornstarch, ketchup, rice vinegar, sugar, lemon juice, and salt.
Bring the mixture to a rolling boil while stirring continuously. Then, lower to simmer and cook until the sauce is slightly thickened.
Add the pineapple and sautéed veggies,stir to combine, then cook for about 30 seconds.
Finally, place in the cooked tofu and toss everything together until well combined and season with salt.
Transfer to a serving bowl and serve warm.