In a saucepan of boiling salted water, cook potatoes just until tender, about 10 minutes. Using a slotted spoon, remove potatoes and let cool enough to handle.
Meanwhile, cook beans in same pot of boiling water until tender-crisp, 2 to 3 minutes. Drain and cover with enough cold water to cool. Drain again.
Divide dressing among 6 Mason jars. Build layers, beginning with potatoes, then green beans, tomatoes, red onion, tuna, eggs, olives and artichokes, dividing each among the jars.
Finish with arugula, lightly packing as much as can fit in remaining space. Seal with lid and keep refrigerated for up to 2 days. When ready to eat, shake jar vigorously to distribute dressing before removing lid.
In a bowl (or using a Mason jar with a tight-fitting lid), whisk (or shake) oil, vinegar, mustard, herbs, salt and pepper to blend.
Nutrition Guide Per Serving: 308 calories, 18 g protein, 15 g fat (3 g saturated fat), 27 g carbohydrates, 6 g fibre, 153 mg cholesterol, 200 mg sodium
Note: You’ll need six (1 L) Mason jars for these main-course-size salads.