Melt butter in a large pot over medium-high heat. Saute onion until translucent, about 5 minutes. Pour in vegetable stock and bring to a boil.
Reduce heat to medium-low. Combine yuca, Yukon potatoes, and red potatoes in the pot; simmer until softened, about 15 minutes. Stir in plantains and corn, simmer until tender, about 20 minutes. Add green onion, lemon juice, 1/2 of the cilantro, garlic, and paprika. Season with salt and pepper. Simmer soup until flavors are well combined, about 5 minutes.
Pour soup in serving bowls and garnish with avocado and the remaining cilantro.