Measure out 1/4 cup of avocado cubes and set remainder aside. Puree the 1/4 cup avocado with 1 cup Italian Dressing (see below) in a blender.
Combine mushrooms, remaining avocado cubes and onion rings in a small shallow dish. Pour pureed dressing over top and chill for one hour.
Meanwhile, prepare Walnut Croutons: In a small skillet, melt butter over low heat. Add salt and walnut pieces. Stir and cook over medium heat until nuts are lightly toasted. Set aside to cool.
When ready to serve, pat spinach leaves dry and place in a layer on a serving platter. Drain the chilled vegetables, reserving dressing. Spoon vegetables over spinach. Arrange orange slices and cheese over top.
Serve with remaining dressing and garnish with walnut croutons.
Combine all dressing ingredients in a blender. Process until well mixed.