In a large skillet over medium heat, sauté the onions in ¼ cup (60 ml) of olive oil, stirring often, for 15 minutes.
Season with salt and pepper, reduce the heat to medium-low, and add the garlic and leek. Continue to cook, stirring often, until the onions are very golden and tender, 15 minutes.
Stir in the asparagus and scallions and continue to sauté for 5 minutes. Stir in the peas and half of the lemon zest and cook 1 minute. Remove the pan from the heat, and set aside to cool completely.
Preheat the oven to 375°F (190°C).
In a large bowl, whisk together the eggs, ricotta, Parmesan, and remaining lemon zest. Season with salt and pepper. Fold in the cooled vegetable mixture.
In a small bowl, whisk together the remaining 6 tablespoons of olive oil and the melted butter. Place one phyllo sheet on a work surface (make sure to cover the remaining phyllo sheets with a slightly damp paper towel to prevent them from drying out).
Use a pastry brush to lightly brush the sheet with the oil-butter mixture. Top with another sheet and brush again. Repeat until you have 5 sheets stacked, finishing with the butter-oil mixture.
Cut the stack of phyllo into four equal rectangles.
Repeat the stacking and cutting with the remaining phyllo sheets.
Brush eight 8-ounce (225 g) ramekins with the oil-butter mixture.
Press a rectangle of phyllo into each of prepared ramekins (it’s okay if they break a bit), and divide the vegetable mixture among the prepared cups. Fold the edges over the filling, leaving the center slightly exposed. Brush lightly with the oil-butter mixture and sprinkle with flaky salt.
Transfer the ramekins to a baking sheet and bake until the phyllo is golden brown and crisp, 30-35 minutes.