Place 2 cups spinach, pistachios, lemon juice, garlic, pepper flakes, salt and pepper in a food processor; pulse until chopped. Add goat cheese; process until blended. Continue processing while gradually adding oil in a steady stream to reach a spreadable consistency. Refrigerate until serving.
Meanwhile, in a large bowl, combine beef and dates, mixing lightly but thoroughly. Shape into six 1/2-in.-thick patties. Place burgers on an oiled grill rack or in a greased 15x10x1-in. pan. Grill, covered, over medium heat or broil 4-5 in. from heat until a thermometer reads 160°, 4-5 minutes per side. Grill buns, cut sides down, over medium heat until toasted.
Top bun bottoms with remaining spinach, burgers, red onion and bacon. Spread 4 teaspoons goat cheese mixture over cut side of each bun top; place on burger. Cover and refrigerate remaining goat cheese mixture; save for another use.
