Put the tomatoes, onion, and cucumber chunks into a large mixing bowl. Add the oregano, lemon zest and juice, and a good drizzle of olive oil, season well with salt and pepper and mix well. Set aside.
Heat a frying pan over medium-high heat, add the cumin seeds, coriander seeds, mustard seeds, and pepper flakes and toast until they release their aroma and begin to brown a little, about 1 minute, shaking the pan and taking care to not let them burn. Transfer to a mortar and add the 1 teaspoon sea salt flakes. Using a pestle, crush the seeds lightly to break them down just a little-- you want to keep a lot of the texture. Add the crushed seed mix to the tomato salad and mix well.
Line a plate with a double layer of paper towels. Set the same frying pan in which you toasted the seeds over medium-high heat and drizzle in some olive or garlic oil. Once the oil is hot, add the bread cubes and fry on both sides until deep golden, 4-5 minutes. Transfer to a paper towel-lined plate and set aside.
Drizzle a little olive oil into the same pan, add the halloumi slices, and fry until golden brown, 1-2 minutes on each side.
Add the fried bread to the salad and toss well to coat the bread chunks in the dressing. Adjust the seasoning as necessary, then arrange the salad and halloumi on a large platter and sprinkle with mint ribbons just before serving.