1 (8.75 ounce) jar sauteed shrimp paste (such as Kamayan)
5 Thai chiles
sliced into thin rings
1 ½ quarts coconut milk
½ quart water
or more as needed
1 bunch fresh spinach
½ cup bias-sliced scallions
Bring a large pot of salted water to a boil. Drop crabs in and cook until shells turn bright red, 8 to 10 minutes. Plunge crabs into ice water to stop the cooking process. Drain and set aside.
Heat a large wok over medium-high heat and add garlic oil. Stir in garlic and cook until lightly toasted and golden brown, 2 to 3 minutes. Add onion and ginger and saute until onion is translucent, 4 to 5 minutes. Add scallion whites. Season with a few pinches of salt and pepper.
Add fish sauce and let infuse for 1 minute. Stir in shrimp paste and toast for about 3 minutes. Add Thai chiles; saute for 1 minute more. Pour in coconut milk and water. Bring to a simmer and reduce heat to medium-low. Cook until sauce is thickened, about 10 minutes, adding more water if too dry.
Meanwhile, break open the crab. Add to the sauce along with any accumulated juices. Simmer until flavors meld, 8 to 10 minutes more. Season with salt and pepper. Toss in spinach and let wilt. Garnish with scallion greens.