In a small bowl, mix the corn flour with 2–3 tablespoons of water to form a smooth slurry. Set aside.
Heat oil in a saucepan over medium heat.
Add the chopped mushrooms and sauté for 2–3 minutes, until they soften and release their moisture.
Remove the mushrooms from the pan and set aside.
In the same pan, add the bell pepper and sauté until crunchy, then set aside.
In the same pan, add the sliced tofu and sauté for 2–3 minutes.
Add the chopped ginger and the sautéed mushrooms back to the pan.
Pour in the water and bring to a gentle boil.
Add the tofu and bell pepper.
Reduce the heat and simmer for 5 minutes.
Slowly add the corn flour slurry while stirring continuously.
Add salt and black pepper.
Cook for 2–3 minutes, until the soup thickens.
Turn off the heat and stir in the lemon juice. Serve hot.
Garnish with chopped cilantro while serving.