Melt the butter in a large pot or Dutch oven over medium heat. Add the onion and celery and cook, stirring occasionally, for 5 to 8 minutes, or until softened. Add the flour and cook, stirring, for 1 to 2 minutes, or until the flour is lightly golden. While stirring, slowly pour in the broth. Add the potatoes, carrots, garlic, thyme, salt, smoked paprika, and several grinds of pepper. Simmer for 25 to 30 minutes, or until the potatoes are tender. Remove the thyme bundle and use an immersion blender to blend the soup until it is mostly smooth but still has some larger potato chunks. If the soup is too thick to blend, add water, ¼ cup at a time, as needed to reach your desired consistency. Alternatively, allow the soup to cool slightly and transfer half to an upright blender. Blend until smooth, then return to the pot with the remaining chunky soup and stir to combine. Add the cream and gently simmer for 5 minutes. Remove from the heat and add the lemon juice, if using. Season to taste and serve with desired toppings.