In a small bowl mix together the shallot, Dijon mustard and vinegar. Add a pinch of sea salt and a little grind of pepper. While whisking slowly pour the oil in until the dressing starts to thicken a little and emulsify. Stop and taste the dressing before adding the remaining oil. You may not need all of the oil depending on the flavor of the vinegar. Add more as necessary.
In a large salad bowl toss together the lettuce, cucumbers and croutons with a little of the dressing. Just until the greens have a shimmer of dressing on them. Season with sea salt and pepper, toss again, taste and adjust dressing and seasonings. Add berries and avocados and drizzle with a little of the dressing.