1. Preheat the oven to 400°F (200°C). Heat 2 tbsp olive oil in an ovenproof frying pan over high heat. Season the beef with salt and pepper. Place in the pan and cook, turning once, for about 3 minutes, until seared on both sides. Transfer the pan to the oven and roast for about 10 minutes, until an instant-read thermometer inserted in the center of the roast reads 130°F (55°C). Let cool. Carve the beef into small strips.

2. Meanwhile, place the tomatoes, cut-side up, on a baking sheet. Sprinkle with the remaining 1 tbsp oil, sugar, and some salt and pepper. Roast for 30 minutes.

3. To make the mustard mayonnaise, combine the mayonnaise and the mustard in a bowl and mix well.

4. To assemble the croustades, spoon the mayonnaise mustard into the croustade baskets. Top each with the beef and 2 cherry tomato halves. Sprinkle with chives and serve immediately.