First make the crespelle: Place the flour in a large mixing bowl. Crack the eggs over the flour and whisk them in. Add salt and whisk in milk, a little at a time, until all milk is incorporated. Allow the batter to stand for 20 minutes.
Meanwhile, melt the butter in a 12 to 14-inch saute pan and add the peas. Cook slowly over medium heat until quite soft, about 15 minutes. Allow peas to cool. Stir the peas into the ricotta.
Heat a 6-inch, non-stick pan until hot and brush with olive oil. Reduce heat to medium and pour 1 1/2 to 2 tablespoons batter into the pan. Cook 20 to 30 seconds, until pale golden, and then flip. Cook on other side 20 seconds and remove to a plate. Continue the process until all the batter has been used, yielding between 17 and 19 crespelle. Fill each crespelle with 3 tablespoons of the cheese ricotta and fold in half. Continue filling crespelle until all are full.
Preheat the oven to 450 degrees F.
Butter the bottom and all sides of 10 by 8-inch ceramic baking dish. Lay the filled crespelle in an overlapping layer in the baking dish. Grate the Parmigiano-Reggiano over the crespelle, dot with the remaining butter and place in oven for 12 to 15 minutes, until piping hot and crispy on top. Remove and serve hot, or allow to rest and serve at room temperature.