Wash the collard greens, then cut out the stems. Stack the greens and roll up like a burrito, then cut into ¼-inch strips.
Chop the bacon, then cook in a medium nonstick pan over medium heat for 5–6 minutes, until crispy. Remove from the pan with a slotted spoon, leaving the rendered fat behind, and drain on a paper towel-lined plate, then cut into ½-inch pieces.
Add the garlic to the pan with the bacon fat and sauté until fragrant, about 1 minute. Add the red pepper flakes, mustard, maple syrup, and apple cider vinegar. Stir to combine, then cook for 2–3 minutes, until slightly thickened. Return the bacon to the pan and stir to combine.
Pour the warm dressing over the collard greens in a large bowl and toss to combine. Transfer to a serving dish and top with the red onion, cucumber, and tomatoes. Serve the salmon alongside, if desired.