For the beef, mix the garlic, thyme and olive oil with some black pepper. Rub over the beef and leave for at least 1 hr, the longer the better. Heat oven to 200C/180C fan/gas 6, season the beef with salt and place in a roasting tin. Roast in the oven for 30 mins, then turn the heat down to 180C/160C fan/gas 4 and cook for 10-15 mins per 450g depending on how you like your meat – 10 mins per 450g will give you rare meat and 15 mins will give you meat that is cooked through.
When the beef is cooked, take it out of the roasting tin and allow it to rest somewhere warm, loosely wrapped in foil, for 30 mins.
Meanwhile In a large pan over medium-high heat, warm the butter and olive oil. Add the onions and salt. Cook until they become tender and have reduced in volume by half, about 10 minutes, stirring frequently to prevent burning. Stir in the vinegar, sugar, and currants. Reduce the heat to medium-low and cook until the onions become jam-like and most of the liquid has evaporated, about 35 minutes. Remove from the heat and let cool to room temperature. The jam will thicken as it cools. (The jam may be made up to 2 days ahead. Cover and refrigerate.
Lightly brush one side of the baguette slices with oil and grill them over direct medium heat until toasted, 1 to 2 minutes on each side. Allow to cool.
Just before serving, slice the beef on a chopping board. Top the crostini with the onion jam, slices of beef, chunks of cheese.