Puree everything in a food processor until smooth and set aside.
For the tacos:
In a large bowl, marinate the steak strips in ⅓ of the puree, in a fridge, for 2-8 hours.
Shake off the excess marinade and cook uncovered in a skillet over medium-high heat, 10 to 15 minutes, until beef is as tender as you like and the mixture is thickened, stirring occasionally.
Divide strips, pineapple, lettuce, cucumber, and onions among the tortillas.
Drizzle the pineapple sauce over the tacos when serving.