Bring a large pot of salted water to a boil. Cut or tear leaves away from thick stems and slice into ½-inch ribbons. Add greens to the boiling water and blanch for 3 minutes. Drain, run under cold water to cool, then squeeze dry.
Place bacon in a large skillet over medium-high heat. Cook until crispy and browned, 8-10 minutes. Use a slotted spoon to transfer bacon to a paper-towel lined plate.
Reduce heat to medium, pour off all but 1 tablespoon bacon grease and add 2 tablespoons butter. When melted, add onion and cook until softened, about 7 minutes. Stir in garlic and cook until fragrant, about 30 seconds. Add remaining 4 tablespoons butter, collard greens, season with salt and pepper, and toss to coat. Cook, stirring every so often, until tender and flavorful, about 10 minutes.
Stir in reserved bacon and pistachios and transfer to a platter. Serve immediately.