Heat canola oil in a large saucepan over medium heat.
Sauté the onion and garlic for about 3-5 minutes until they become translucent.
Add in the diced rhubarb and water. Cook for about 5 minutes until the rhubarb starts to soften.
Stir in the brown sugar, tomato paste, cider vinegar, Dijon mustard, chili powder, cayenne pepper, Worcestershire sauce, salt, pepper, and liquid smoke or smoked paprika if using.
Bring the mixture to a boil, then reduce the heat and let it simmer for 30 to 45 minutes until the rhubarb breaks down and the sauce thickens.
If you prefer a smooth sauce, puree it using an immersion blender or food processor.
Taste the sauce and adjust the seasoning as needed. You can add more sugar for sweetness, vinegar for tanginess, or cayenne for extra heat.
Notes
Store the sauce in the refrigerator for up to 7-10 days or freeze it for up to 6 months.