To make the croûtons, toss the bread pieces in the oil to coat. Spread out in a single layer on a baking tray and toast for about 18–22 minutes, turning after 10 minutes so they brown evenly. Bake until golden brown and crunchy. Season with salt and pepper and leave to cool.
For the red wine vinaigrette, place the shallot and vinegar in a bowl for 15 minutes to infuse. Add a generous pinch of salt, a small pinch of pepper and the oil. Stir to combine.
For the panzanella, toss the onion with the vinegar in a small bowl and leave for 20 minutes to infuse.
Bring a large saucepan of salted water to the boil. Blanch the asparagus for about 3½ minutes, or until just tender. Drain and leave to cool on kitchen paper.
Place half the croûtons in a large salad bowl and toss with half of the vinaigrette. Leave to sit for about 10 minutes.
Add the remaining croûtons, asparagus and infused onions to the salad bowl. Add the mint leaves and crumble in the feta. Dress with the remaining vinaigrette, season with salt and pepper, toss and serve.