Instructions:
- Melt 3 Tbsp butter in a large, heavy-bottomed pot over medium heat. Add onion, carrots, celery, and leek. Sauté gently for 8–10 minutes until soft and translucent (sweated, not browned). Add garlic and thyme; cook for 1 minute until fragrant.
- Clear a space in the centre of the pot. Add the remaining 3 Tbsp butter. Once melted, whisk in the flour until a smooth paste (the roux) forms. Cook this paste for 1-2 minutes, stirring constantly, to cook out the raw flour taste.
- Gradually whisk in the warm stock, ensuring no lumps form. Bring the mixture to a gentle simmer, stirring occasionally.
- Once simmering, reduce heat to low, cover partially, and cook for 15–20 minutes, or until the hard vegetables (carrots) are fork-tender.
- Stir in the milk/half-and-half, salt, and pepper. Bring the soup back to a very low simmer (do not boil after adding milk).
- Remove the pot from the heat entirely. Gradually add the grated Cheddar and Gruyère, stirring constantly until every last bit of cheese has melted into a smooth, glossy sauce.
- Taste and adjust seasoning (salt/pepper). Serve immediately, perhaps garnished with a drizzle of olive oil and fresh black pepper.