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Optional, sauté onions, carrots, celery, and garlic until onions are at least translucent. This will give your vegetables a head start softening and bring another layer of flavor.
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Combine all ingredients except the milk and noodles, in the slow cooker.
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Cook on high for one hour and low for two hours.
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In the last hour of cooking stir in the milk and noodles.
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The tetrazzini is done when the spaghetti noodles are softened and the dish is heated thoroughly.
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If the tetrazzini is too soupy, turn the crock-pot on high with the lid off, to reduce some of the liquid. It thickens up in about 15 minutes.