In a large heavy-bottomed pot (like a Dutch oven), melt the butter over medium heat. When the butter is melted, stir in the onions, carrots, and celery. Cook, stirring occasionally until they begin to soften; 5 to 6 minutes.
Stir in the garlic, rosemary, red pepper flakes, salt, and black pepper. Continue stirring everything around the pot.
Scatter the flour over the vegetables and cook, stirring, until it looks medium blonde in color; about 1 minute.
Slowly whisk in the broth, making sure there aren’t any lumps of flour until blended. Increase the heat and bring the soup to a boil, then reduce it to a simmer.
Add the potatoes and bay leaf, and then cook, partially covered with a lid, until the potatoes are fork tender; about 20 minutes.
Turn the heat to low, remove the bay leaf, and then stir in the cream and cheese.
Taste for seasoning. Adjust with more salt and pepper as needed to taste.
Toast the bread. Then spread with butter.
Mix the sparkling water and lemon slices. Serve over ice if desired.
Portion the soup and garnish with chives. Serve with bread and spritzer.