1. 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.
2. Add 1 3/4 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.
3. Arrange pimiento strips over rice; sprinkle with parsley. Serve immediately.