Soak the rice noodles in a baking dish filled with water for 10 minutes to soften. Set aside. Bring a pot filled with water to a boil. See note below on using the water to cook the shrimp, if not cooked already.
In a large sauté pan or skillet, add olive oil over medium heat. Brown the ground pork for 4-5 minutes. Add the onion, garlic, and fish sauce. Season with salt and pepper to taste. Cover over low heat.
Once the water from the pot is boiling, put the soaked noodles in the water and cook for 3-4 minutes until softened. Strain the noodles and set aside.
Dissolve the achuete annatto powder in 4 cups of water. Once dissolved, add the water to the pan with the meat. Bring to a boil. Once the water is boiling, add the shrimp cubes. Simmer for 5 minutes.
In a small bowl, mix the ½ cup of water with a ½ cup of cornstarch until a slurry forms. Add the mixture to the pan and combine with the sauce until thickened to desired consistency. Stir consistently. Add the lemon juice and season with salt and pepper to taste. Turn off the heat. (Optional - Mix another ½ tablespoon of cornstarch with a ½ tablespoon of water to add to the pan if you would like the sauce thicker.)
Remove half of the sauce from the pan and set aside. Add the cooked noodles to the pan and mix well. When serving, spoon the other half of the sauce on top of the noodles. Add the toppings on top of the noodles. Enjoy!