Heat a grill pan over high heat until very hot.
Sprinkle each slice of watermelon with salt and a pinch of sugar and grill quickly on each side, just until grill marks show and the fruit is lightly caramelized, about 1 minute per side. Arrange them on serving plates and sprinkle with crumbled feta, and salt and pepper, to taste. In a small bowl, toss the arugula and fresh herbs with olive oil, vinegar, lime zest, salt and pepper, to taste. Put small mounds on top of the grilled watermelon and feta. Just before serving, heat a small skillet over medium-high heat and quickly saute Halloumi until golden and crispy. Garnish each plate with the Halloumi cheese croutons and serve immediately.