In a large steamer pan, steam broccoli and peas until just heated through (heat to 165°F). Set aside to cool.
While vegetables are cooling make the dressing. Combine in a saucepan over medium heat: vinegar, butter, sugar, flour, salt and mustard; bring to boil. Continue to cook for 5 minutes stirring frequently. Cool completely.
In a large bowl combine steamed vegetables, tomato and avocado. Toss to combine.
Pour cooled dressing over bowl of vegetables. Toss to distribute dressing evenly. Cover tightly and store in refrigerator. Serve topped with feta cheese.
