Begin by heating olive oil in a skillet over medium heat. Add the chopped onion and minced garlic, sautéing until the onion is translucent and fragrant, about 3-4 minutes. Transfer the cooked onion and garlic into the slow cooker.
Add the diced sweet potatoes, canned tomatoes, cumin, smoked paprika, and cayenne pepper to the slow cooker. Pour in the vegetable broth and stir gently to combine everything evenly.
Cover and cook on low for 6 hours, or until the sweet potatoes are tender and the flavors have melded, filling your kitchen with a warm, spicy aroma.
About 30 minutes before serving, stir in the drained black beans and kidney beans. Let the chili simmer uncovered with the lid slightly ajar to thicken if needed.
Check and adjust seasoning as desired, adding salt, pepper, or a splash of lime juice for brightness. The chili should be thick, chunky, and aromatic, with the sweet potatoes slightly melting into the broth.
Serve hot, garnished with fresh herbs or a drizzle of olive oil if preferred. Enjoy the cozy, hearty goodness of this slow-cooked sweet potato chili.