Prepare the onions: Halve each onion from root to tip, then peel and slice into 5 mm half-rings. You'll hear a satisfying crunch as the knife goes through — that's the sound of a good, fresh onion. The pile will look enormous; don't panic, they shrink dramatically.

Caramelise on the stovetop: Melt the butter and olive oil together in a large frying pan over medium-high heat. Add the onions, salt, pepper, and sugar. Stir to coat. Cook for 20-25 minutes, stirring every few minutes. You'll see the onions go from opaque white to translucent, then slowly turn a deep golden brown. The smell will become sweet and almost jammy. If the bottom of the pan develops a dark crust (called fond), that's flavour gold — don't scrub it off.

Deglaze the pan: Once the onions are a rich amber colour, add the balsamic vinegar and minced garlic. Stir for 30 seconds — you'll hear a satisfying sizzle and the vinegar will release any stuck bits from the pan bottom, creating a glossy glaze on the onions.

Thicken the base: Sprinkle the flour over the onions and stir continuously for 2 minutes. The mixture will look pasty and clumpy, and you'll smell a toasty, nutty aroma as the flour cooks. This step prevents a watery soup later.

Transfer to the crockpot: Scrape every bit of the onion mixture into your slow cooker. Pour in the beef broth, add the thyme sprigs and bay leaves. Give everything a good stir — the liquid should be a rich, dark brown colour with strands of caramelised onion floating through it.

Slow cook: Cover and cook on LOW for 6 hours (or on HIGH for 4 hours). After the first hour, your kitchen will smell like a cosy bistro. The soup will deepen in colour and the flavours will meld into a savoury-sweet harmony. Remove the thyme stems and bay leaves before serving.

Prepare the cheesy toasts: About 10 minutes before serving, preheat your grill to high. Arrange the baguette slices on a baking tray and toast for 2-3 minutes per side until golden and crisp. You'll hear a hollow sound when you tap them — that means they're ready.

Assemble and finish: Ladle the soup into oven-safe bowls, leaving about 2 cm of space at the top. Float two baguette slices on each bowl, then pile on the Gruyère and a sprinkle of Parmesan. Place the bowls under the grill for 3-4 minutes until the cheese is bubbling, golden, and slightly charred in spots. Watch closely — it goes from perfect to burnt very quickly. Serve immediately.