Caramelized Onion and Fennel Soup with Thyme

By AislePrompt Editorial · Published 2026-05-12 · Updated 2026-05-12

A fragrant, slow-simmered soup featuring tender caramelized onions and sweet fennel, accented by fresh thyme for depth. This French-inspired soup recipe (vegetarian), ready in about 85 minutes, pairs yellow onions, fennel bulb, unsalted butter for a weeknight-friendly dinner that comes together with one pan and minimal cleanup. Each serving lands at about 120 calories and feeds 6, so it slots cleanly into a weekly meal plan and pairs well with a quick salad or grain on the side. Tap "Plan a meal with the AI" below to drop this recipe into your week and send the ingredients straight to Instacart. Nutrition values are approximate, calculated from USDA FoodData Central (fdc.nal.usda.gov) — verify against ingredient labels for any health-driven dietary plan.

Ingredients

• 4 large (about 2 lbs) yellow onions
• 1 medium fennel bulb
• 3 tbsp unsalted butter
• 2 tbsp olive oil
• 3 fresh thyme sprigs
• 6 cups vegetable broth
• 2 tsp salt
• 1 tsp black pepper
• 1 bay leaf

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Instructions

1. Step 1: Thinly slice 4 large yellow onions and 1 medium fennel bulb into 1/8-inch strips. Heat 3 tbsp unsalted butter and 2 tbsp olive oil in a large pot over medium heat.
2. Step 2: Add the sliced onions and fennel along with 3 sprigs fresh thyme and 1 bay leaf. Cook gently, stirring occasionally, for 45 minutes until the vegetables are deeply golden and caramelized.
3. Step 3: Pour in 6 cups vegetable broth, season with 2 tsp salt and 1 tsp black pepper, and bring to a simmer. Cook uncovered for 20 minutes to meld flavors.
4. Step 4: Remove thyme sprigs and bay leaf, then use an immersion blender to puree the soup until smooth but still slightly textured.
5. Step 5: Serve hot, garnished with a fresh thyme sprig if desired.