Chimichurri: Combine garlic, aji, and salt in a mortar. Add vinegar, water, and drizzle in oil.
Steak: Marinate steak in all but 1 tablespoon chimichurri for about 1 hour, covered and refrigerated.
In a large pot, cover beans with water and cook with bacon and onion, without salt. Allow to simmer until tender, about 1 1/2 hours. Strain the beans, reserving the cooking liquid. Remove and discard the bacon and onion. Put back the cooking liquid on the stove, allow to boil until cooking liquid is starchy and thick. Reserve 1 cup.
Grill steak until medium rare, about 2 to 3 minutes on each side.
Slice steak into 2-inch pieces. Mix into the beans add all the other ingredients, including the reserved chimichurri and reduced cooking liquid. Season and serve at room temperature.