In a large pot, heat up oil to medium then add scallions and ginger and cook until fragrant, about 1-2 minutes. Turn heat up to medium high and add the mushrooms.
Once softened/cooked, add coconut milk, water, red curry paste, turmeric, black pepper and fish sauce. Bring to a rolling simmer then bring down the heat to a medium low and let simmer for 10 minutes.
Add the tofu and let simmer for 10 minutes more.
Remove from heat and stir in the baby spinach. Serve over white rice.