Heat olive oil in a skillet over medium heat. Add chopped onion and cook until translucent, about 4–5 minutes. Add garlic and cook for 30 seconds. Set aside to cool.
Rinse collard greens thoroughly. Remove tough stems by folding each leaf in half and slicing away the center rib. Stack leaves, roll tightly, and slice into ½- to 1-inch ribbons.
Add collard greens to the slow cooker along with the onion mixture, broth, vinegar, soy sauce, and maple syrup. Sprinkle in smoked paprika, red pepper flakes, salt, and black pepper. Toss with tongs to combine.
Cover and cook on high for 3–4 hours or low for 5–6 hours, until collards are very tender. Stir once halfway through if possible.
Taste and adjust seasoning before serving.
Notes
Be sure to clean the collard greens thoroughly to remove any dirt. Otherwise, they could become gritty.
For a smokier flavor, add ¼-½ tsp of liquid smoke.
Sprinkle on hot sauce after serving for more heat.
For a spicier dish, add a diced jalapeno or a pinch of cayenne.
If the collard greens are looking dry near the end, add ¼-½ cup of additional vegetable broth.
Kale, mustard greens, or turnip greens cook faster than collards. Reduce the cooking time by about 30–60 minutes and check for tenderness early, especially when using the high setting.