heat a splash of neutral oil in a skillet over medium-high heat
add the zucchini in a single layer. once the undersides are lightly browned, give everything a stir and let the new side sit undisturbed.
repeat this another 2–3 times, allowing the zucchini to develop color before moving it around. you're looking for most pieces to be medium to dark brown on at least one side, with some lightly charred edges mixed in. season with salt at the end.
let the zucchini cool for 5–10 minutes (so it doesn't wilt the basil when mixed)
whisk together the garlic, dijon, lemon juice, red wine vinegar, and olive oil until emulsified, season with salt and pepper
slice the tomatoes into rounds ~1/4 inch thick
in a bowl, combine the zucchini, tomatoes, feta, and basil. pour over the dressing and gently toss to coat.
optional: finish with extra feta and more basil