Start by peeling, coring, and chopping the apples into small chunks, setting them aside on a plate.
Dice the onion and mince the garlic, preparing them for sautéing.
Warm the olive oil in a large saucepan over medium heat, then add the diced onion. Cook, stirring occasionally, until the onion becomes soft and translucent, about 5 minutes, letting it soften and release a fragrant aroma.
Add the minced garlic and cook for another minute until fragrant, being careful not to burn it.
Stir in the ground cinnamon and red chili flakes, cooking for 30 seconds to toast the spices and intensify their aroma.
Add the chopped apples to the pot, stirring well to coat them with the spice mixture. Sauté for a few minutes until the apples start to soften slightly.
Pour in the 4 cups of vegetable or apple cider, bringing the mixture to a gentle boil. Reduce the heat to low, cover the pot, and let it simmer for about 20 minutes, until the apples are tender and easily mashable.
Use an immersion blender to purée the soup directly in the pot, blending until smooth and creamy. Alternatively, carefully transfer the hot mixture to a blender and purée until silky, then return it to the saucepan.
Taste the soup and season with salt and black pepper as needed, stirring well to incorporate the flavors.
Heat the soup gently for a few minutes if needed, allowing the flavors to meld, and then pour into bowls.
Finish with a sprinkle of red chili flakes or a swirl of cream for an added touch of richness and spice before serving.