Preheat the oven to 350ºF. Coat a baking sheet with the cooking spray.
In a small bowl, add the potatoes, olive oil, and salt. Toss to evenly coat. Spread the potatoes in a single layer on the baking sheet. Roast until golden brown and tender, 20 to 25 minutes. Set aside to cool for 10 minutes.
While the potatoes are roasting, place the eggs in a medium pot and cover them with water. Place the pot over high heat and bring the water to a boil. Cook the eggs for 3 minutes, then remove the pot from the heat, cover it, and let stand for 15 minutes. Drain and run cold water over the eggs until they are completely cool, about 10 minutes. Peel and slice into quarters lengthwise.
Into each of four glass containers, place 1 cup of the spinach. Top each with ¼ cup of the potatoes, 4 egg quarters, ¼ cup each of green beans and tomatoes, and ½ can of tuna with the oil. Sprinkle each with the black pepper and add a lemon wedge. When ready to eat, squeeze the lemon juice on the tuna.
REFRIGERATE: Store each sealed jar for up to 5 days.