Place the whole Thai green chilis, the red onion and the garlic cloves in a food processor and pulse to blend until smooth. You can also do this with a mortar and pestle.
Bring the water and bouillon cubes to a boil in a large saucepan. Add the chili paste to the boiling water along with the fish sauce, lemongrass, lime leaves, ginger, mushrooms and cherry tomatoes. Bring to a boil, then reduce heat to low, cover and simmer for 20 minutes.
Uncover, add the green onions and shrimp and simmer until shrimp cooks through. Depending on the size of the shrimp, this could take anywhere between 3-6 minutes.
Add the instant ramen noodles, and let them simmer just a minute or so until cooked through. They'll cook fast!