Heat olive oil in a large pot over medium heat until shimmering. Add the chopped onion and sauté until translucent, about 5 minutes, until it begins to soften and smell sweet.
Add the sliced mushrooms to the pot and cook, stirring often, until they release their moisture and start to brown, about 8 minutes. The mushrooms should be tender and darkened.
Stir in the minced garlic and cook for another minute, just until fragrant. Be careful not to burn the garlic to keep it mild and aromatic.
Pour in the apple cider and bring the mixture to a gentle simmer. Let it cook for about 5 minutes to allow the cider to reduce slightly and deepen in flavor.
Pour in the vegetable or chicken broth, then bring the entire mixture to a gentle simmer. Cover the pot partially and cook for 10 minutes to develop the flavors.
Using an immersion blender, carefully blend the soup directly in the pot until smooth and velvety. Chop some of the mushrooms into small pieces if you prefer a little texture, or blend completely for a silky finish.
Stir in the heavy cream and season with salt and pepper to taste. Return to a gentle heat and simmer for another 3-5 minutes until heated through and creamy.
Taste and adjust seasoning as needed, adding more salt, pepper, or a splash of cider for extra brightness. The soup should be warm, flavorful, and smooth.
Pour the hot mushroom and apple cider soup into bowls, garnish with fresh herbs if desired, and serve immediately for a cozy, autumn-inspired meal.