In a small bowl, combine vegetable broth, soy sauce, rice vinegar, maple syrup, cornstarch, sriracha, ginger powder, and minced garlic; whisk until the sauce is smooth.
Warm a wok or a large non-stick skillet over medium heat and add coconut oil. Once melted, stir in sliced onion and sauté until they become translucent, about 2 to 3 minutes.
Add the sliced mushrooms, shredded cabbage, and thinly sliced carrots to the pan. Stir-fry everything over medium-high heat for 5 to 6 minutes, keeping the vegetables moving constantly. Season with salt and freshly ground black pepper.
Stir in the canned lentils along with the prepared stir-fry sauce (give it a quick whisk again before pouring it in), then reduce heat slightly and simmer while stirring for about 2 minutes, allowing the sauce to thicken and coat the ingredients.
Serve the stir-fry hot over cooked rice and garnish with chopped spring onions if available.