Trim the ends off the courgettes and slice them lengthwise into planks about 0.5 cm thick. Thinner slices will collapse and turn mushy on the grill.
Toss the courgette planks with 2 tbsp olive oil and 1/2 tsp salt in a large bowl, then pat lightly dry with a paper towel to help them char instead of steam.
Combine the sliced red onion, red wine vinegar, and a pinch of salt in a small bowl. Let it quick-pickle while you grill.
Toast the pine nuts in an 8-inch skillet over medium heat for 2 to 3 minutes, stirring often, until golden. Tip onto a plate to cool.
Heat a 12-inch grill pan or outdoor grill to medium-high, about 230 C / 450 F. Lightly oil the grates or pan surface.
Grill the courgette planks in batches of 4 to 5 pieces so they don't overlap, 2 to 3 minutes per side, until char-striped and tender-crisp, not mushy.
Transfer grilled courgette to a platter and let it cool for 5 minutes.
Whisk the remaining 2 tbsp olive oil with the lemon zest, lemon juice, grated garlic, and black pepper in a small bowl to make the dressing.
Drain the pickled red onion and add it to the courgette along with the dressing and chili flakes. Toss gently to coat.
Scatter mint, crumbled feta, and toasted pine nuts over the top just before serving. Serve warm or at room temperature.