Heat 2 tbsp oil in medium skillet over medium het. Add zucchini, onion and garlic and saute until tender but not brown, about 5 minutes. Add prosciutto and next 6 ingredients. Saute 1 minutes. Remove from heat. Cool. Season to taste with salt and pepper. Mix in yolk. Using melon baller, scoop out centers of squash, leaving generous 1/4" thick shell. Blanch squash shells in pot of boiling salted water 3 minutes. Drain; cool. Cut thin slice from rounded end of tomato halves if necessary to stand flat. Lightly brush baking sheet with olive oil. Fill vegetables using about 1 heaping tbsp filling for each tomato half and 1 heaping tsp for each squash. Arrange vegetables on baking sheet. (Can be prepared 1 day ahead. Cover and refrigerate.) Preheat oven to 400?. Bake vegetables uncovered until tneder, about 30 minutes. Transfer to platter. Serve warm or at room temperature. Posted to EAT-L Digest 03 Sep 96 Date: Wed, 4 Sep 1996 07:50:43 -0400 From: Betsy Burtis