In a pan, add enough water to fill 1/4 inch across the bottom. Add spinach and cover, steam over medium to med-high heat for four minutes until tender. Drain spinach well, removing all of the water by placing in a strainer and lightly pushing down. In a medium size bowl combine cooked spinach, parmigiano-reggiano cheese, lemon juice, lemon zest and 1/2 cup yogurt cheese and gently fold. Add salt and pepper to taste. In a medium size bowl combine 1/2 cup yogurt cheese and gruyere and gently fold. Preheat oven to 400°. Place beets, carrots, zucchini and mushrooms on a sheet pan. Brush with oil and sprinkle with salt&pepper. Roast for 12-14 minutes until golden brown. Reduce oven heat to 325°. Roll out dough on floured surface to about 14x10 inch square. Spray pan with cooking spray and press dough into bread pan with edges hanging over about 3 inches all around. Layer ingredients, starting with of the mushrooms and zucchini, then add a layer of spinach mousse, beets, carrots, yogurt cheese, and another layer of remaining roasted vegetables. Fold dough to cover edges, whisk egg and brush over the top. The top will become the bottom of the terrine once baked, so seal well with the egg wash. Bake terrine for 35-40 minutes. Let cool and turnover onto serving dish.