Season the beef generously with salt and pepper.

Heat a tablespoon of olive oil in a large pot over high heat and sear half the beef slices for 2 minutes per side until golden brown. Transfer to a plate and repeat with the remaining beef, adding more oil if necessary.

Deglaze the pot with a splash of broth, scraping up any brown bits, and pour the liquid over the beef.

In the same pot, melt butter and olive oil over medium-high heat, add onions with a pinch of salt, and cook, stirring frequently for 8-10 minutes until golden.

Stir in garlic, brown sugar, and thyme, then lower the heat and let the onions caramelize for another 30 minutes until soft and golden.

Preheat the oven to 350°F (170°C) and brush both sides of the baguette slices with olive oil. Bake for 20 minutes, flipping halfway through until crisp.

Set aside a spoonful of caramelized onions for garnish. Increase the heat, sprinkle flour over the remaining onions, and stir for a minute.

Gradually whisk in wine and broth, then add bay leaves and seasonings. Return the beef to the pot, submerging it in the sauce, bring to a boil, cover, and transfer to the oven for 2 hours.

In a bowl, mix eggs, mustard, cream, and Gruyère, then spread the cheesy mixture onto the croûtes, arranging them overlapping on top of the beef.

Increase the oven temperature to 400°F (200°C) and bake uncovered for 20 minutes until bubbly and golden. Garnish with reserved onions and fresh thyme before serving.