Place a large Dutch oven or heavy-bottomed pot over medium heat and melt the butter or heat the olive oil.
Add the diced onion, carrots, and celery to the pot and sauté until softened, about 5 to 7 minutes.
Stir in the minced garlic, rosemary, and red pepper flakes, cooking until fragrant for about 1 minute.
Sprinkle the flour over the vegetables and stir well to coat, cooking for 1-2 minutes to remove the raw flour taste.
Slowly pour in the vegetable or chicken broth while whisking constantly to prevent lumps.
Add the cubed potatoes and the bay leaf, then bring the mixture to a gentle boil.
Reduce the heat to low, cover, and simmer for 15 to 20 minutes, or until the potatoes are fork-tender.
Remove the bay leaf, stir in the heavy cream or yogurt, and season with salt and pepper to taste.
Optional: Use an immersion blender to partially blend the soup for a thicker texture if desired.
Serve hot, garnished with shredded cheese and fresh chives or scallions.