Add the lemon juice, wholegrain mustard, dijon mustard, garlic, maple syrup, sumac, salt and pepper to a medium bow. Whisk to combine.
Slowly pour in the olive oil while whisking. Continue whisking until all of the olive oil is incorporated into the lemon mixture and the dressing is smooth and silky.
Store the salad dressing in a glass airtight container or glass jar with a lid, in the fridge for up to one week. Bring the dressing to room temperature before using.