In large skillet, heat 1 Tbsp of oil over medium high heat.
Add onion and leeks and cook, stirring occasionally, until onion is translucent, about 5 minutes. Transfer to large mixing bowl, reserving the skillet.
Add another 1 Tbsp of oil to the skillet and reduce heat to medium. Add carrot and cook, stirring occasionally, until tender, about 3 minutes. Transfer to onion bowl, reserving the skillet.
Add remaining 1 Tbsp of oil to the skillet over medium heat. Add cabbage and cook, stirring constantly, until lightly wilted, about 3 minutes. Transfer to onion bowl, reserving the skillet.
Add vinegar and maple syrup to the skillet over medium heat, stir to combine until mixture starts to bubble, about 30seconds.
Pour mixture over the slaw, add parsley, salt, and pepper and mix gently until thoroughly combined.