TO PREPARE VEGETABLE LAYERS:
In small bowl, combine dry mushrooms and hot water to cover. Let stand 30 minutes. Drain.
While mushrooms are rehydrating, bring large pot of water to boiling. Blanch vegetables in separate batches in boiling water. Blanch cabbage leaves 3 minutes, spinach leaves 1 minute, asparagus and red pepper strips 3 minutes, and carrots 4 minutes. Plunge vegetables into ice water after blanching. Drain.
Pat all blanched vegetables, mushrooms, and artichokes dry between 2 to 3 layers of paper towels, applying gentle pressure to squeeze out as much liquid as possible. (spinach leaves should be opened out flat to assume original shape)
TO MAKE BATTER:
In medium bowl combine eggs, 1/2 cup buttermilk, parsley, thyme, tarragon, shallots, and salt and pepper to taste. Whisk until blended. (This batter is overseasoned because it will flavor all the vegetables)
TO ASSEMBLE TERRINE:
Generously butter 9x5x3 inch loaf pan. Using a triangular-shaped cut, remove 1 to 2 inches of hard white stem from cabbage leaves to make leaves more flexible.
Line sides, then bottom of pan with cabbage leaf to make leaves, overlapping the leaves generously to form a "leakproof shell" and allowing leaves to extend well over the rim of the pan. (The tops of leaves will be folded over the top of the finished terrine.) Pour 1/2 cup batter over cabbage in bottom of pan.
Arrange asparagus spears lengthwise in pan, packing spears tightly together. Add 2 layers of red pepper strips placed crosswise in pan. Pour in a little more batter, pressing peppers down gently.
Layer mushrooms over peppers. Layer carrots lengthwise in pan. Pour in a little more batter, pressing carrots down gently. (Pressing each layer down will raise the level of the batter)
Layer half the spinach leaves over carrots. Add a layer of artichoke hearts over spinach, then add a little more batter. Gently press down artichoke hearts.
Top with a layer of remaining spinach leaves, add remaining batter and pressing down each layer. Fold ends of cabbage leaves over top of terrine.
Preheat oven to 350 degrees F. Cover loaf pan with buttered parchment paper cut to fit inside edge of pan. Place loaf pan in 13x9x2 inch baking pan. Pour hot water around loaf pan to depth of 1 inch.
Bake at 350 degrees F. for two hours until firm. Remove from water bath. Cool on rack (still covered with parchment paper) 2 hours.
Refrigerate, still covered, overnight.
TO MAKE TOMATO SAUCE:
In blender, combine chopped tomato, 1/4 cup buttermilk, tomato paste, cayenne pepper, garlic, and salt and pepper to taste. Blend until smooth.
Loosen edges of terrine with knife. Invert on serving platter. Cut cold terrine in crosswise slices with serrated knife.
Spread a thin layer of tomato sauce on each serving plate. Arrange slices of terrine over sauce.