Preheat the oven to 180 degrees Celsius.
Lay the chopped strawberries for the dressing in a baking tray. Roast them for 15 minutes.
Once roasted, remove the tray from the oven and tip the strawberries into a bowl. Allow them to cool.
Heat a small saucepan over medium heat and add in the walnut pieces. Toast them for around 5 minutes until they're fragrant. Leave them to cool.
Add the cos lettuce, feta, onion, fresh strawberries, walnuts and fresh herbs to a large salad bowl and toss them all together. Reserve a little bit of the feta, onion, walnuts and strawberries to sprinke on top for decoration at the end.
Using a stick blender, blitz together the cooled strawberries along with the salt, mustard, balsamic and oil. Taste and season more if necessary.
Drizzle 4-5 tablespoons of dressing over the salad and toss it together. Sprinkle over the remaining reserved salad ingredients and an extra drizzle of salad dressing.
Any remaining dressing can be stored in a jar in the refrigerator for up to a week.