Season the beef generously with salt and freshly cracked black pepper.
Set the Instant Pot to Sauté. Add the olive oil and brown the beef in batches. Transfer to a plate.
Add the butter and onions. Cook for 20–25 minutes, stirring regularly, until deeply softened and caramelised.
Add the garlic and cook for 30 seconds.
Pour in the red wine and scrape the bottom of the pot thoroughly.
Add the beef stock, Worcestershire sauce, balsamic vinegar, Dijon mustard, bay leaves and thyme.
Return the beef to the pot and cook on High Pressure for 45 minutes.
Allow a 15-minute natural pressure release, then carefully release the remaining pressure.
Remove the bay leaves and thyme stems.
Combine the cornflour and cold water. Stir into the casserole and simmer on Sauté until the sauce thickens slightly.
For the toasties, butter one side of each slice of bread. If desired, spread a small amount of Dijon mustard on the inside.
Fill with grated gruyère and assemble with the buttered sides facing out.
Cook in a frying pan or sandwich press until golden outside and melted inside.
Cut the toasties into halves or triangles and serve alongside the casserole for dipping.
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