Heat oil in a large pot over medium heat. Add minced garlic and cook until fragrant, about 30 seconds. Add diced onions and cook until soft and clear, about 2-3 minutes.
Add the sliced pork belly and cook until it starts to brown, about 5-7 minutes. Stir occasionally to prevent sticking.
Pour in the coconut milk and pork stock. Add ginger and lemongrass if using them. Let the mixture come to a gentle simmer and season with salt and pepper to taste.
Let everything simmer for 15-20 minutes until the pork becomes tender. Keep the heat at medium-low to prevent the coconut milk from curdling.
Add the cubed kalabasa (squash) to the pot and cook for 5 minutes. The squash should be almost tender but not falling apart.
Add the cut sitaw (snake beans) and cook for another 3-4 minutes. The beans should stay crisp-tender and bright green.
Add the peeled shrimp and cook just until they turn pink, about 2-3 minutes. Don't overcook the shrimp or they'll become tough.
Taste and adjust the seasoning. Remove the lemongrass stalks if you used them.
Let the dish rest for 5 minutes before serving. This helps the flavors come together and lets the sauce thicken slightly.
Serve hot with steamed white rice. Place patis (fish sauce) and calamansi halves on the side for people to add to taste.
For leftovers, let the dish cool completely before storing in an airtight container in the refrigerator. It will keep well for up to 3 days.