Preheat the oven to 200°C/400°F.
Place thyme sprigs, garlic, sea salt, and black pepper in a roasting tin. Drizzle with olive oil and toss together until combined. Place a fillet of beef on top of the mixture. Roast for 25 minutes or until golden brown and cooked to your liking.
Meanwhile, prepare the mustard mayonnaise by mixing together 5 tsp djon mustard and 5 tbsp mayonnaise in a small bowl.
For the spicy tomato relish, heat some olive oil in a large frying pan over medium heat. Add the sherry vinegar, shredded basil leaves, and red chilies, followed by the halved cherry tomatoes and chopped red onion. Fry for 7-8 minutes or until softened and reduced in size slightly.
Heat some olive oil in a large frying pan over medium-high heat. Once hot, add the steak and fry for 2-3 minutes on each side or until cooked to your liking. Transfer to a plate and leave to rest for 5 minutes before slicing into thin strips.
Toast the ciabatta slices in a clean frying pan over medium heat with some olive oil for 1-2 minutes on each side or until golden brown.
To assemble the sandwich, spread some mustard mayonnaise onto each slice of ciabatta bread. Top with lettuce, steak strips, and a generous spoonful of spicy tomato relish. Serve immediately!