Heat oil in 10”-12” paella pan (or large skillet) over medium-high heat. Add onions and chorizo; cook until onions are tender and chorizo turns golden brown, about 7 minutes. Stir in garlic; cook until fragrant, about 30 seconds more.
Add 1 ¾ cups water to pan; bring to boil. Stir in contents of yellow rice mix and alcaparrado. Boil rice mixture 1 minute; tightly cover pan. Reduce heat to medium-low; simmer until rice is tender and water is absorbed, about 25 minutes, adding shrimp and clams after 12 minutes.
Arrange pimiento strips over rice; sprinkle with parsley. Serve immediately.
To rid clams of crunchy sand, try this soaking technique. First rinse them under cold water. Then, place rinsed clams in bowl filled with cold water and a few spoonfuls of coarse salt; transfer to refrigerator and soak 30 minutes. During this quick soak, the clams take in the salt while purging any grit in their digestive system. Rinse again, then proceed.