Soak the rice noodles in water for 15 minutes. Meanwhile, boil enough water in a pot. Once the noodles are done soaking, drain them from the cold water and then submerge in boiling water for a few minutes just until they are cooked but still firm, about 3-5 minutes. Once cooked, drain the water and transfer to a container or large bowl and set aside.
In a saucepan over medium heat, saute garlic and onion in oil until tender. Add the ground pork and fish sauce and cook for 5-10 minutes or until all bits are cooked. Stir often to get rid of big lumps.
Dilute the annatto powder in the 4 cups of water and add this to the pork. Add the shrimp bouillon and let simmer for some minutes until it starts to boil.
In a small bowl combine flour and 1 cup of water to make a slurry. Add this to the pot and stir until the sauce becomes thick. Season with ground pepper and then turn off the heat.
Pour three-quarters of the sauce into the noodles and mix until all noodles are covered with sauce. Transfer the noodles to a serving dish. Pour remaining sauce on top. Arrange toppings on top, finishing with the crushed chicharon and spring onions. Place the calamansi or lemon wedges on the sides or in another smaller bowl for squeezing. Serve while warm.
Note 1. Different brands of annatto powder can have varying effects. Some can have stronger coloring effects even with just a tablespoon. Start with a small amount first and add more as needed.