Preheat oven to 350 F.
Arrange broccoli and cauliflower florets on a baking sheet. Drizzle evenly with olive oil.
Roast veggies for about 30 minutes. The edges should be light golden brown.
While vegetables are roasting, make the dressing. In a small food processor, add mayonnaise, avocado, garlic, lemon juice, salt and pepper. Process until smooth.
Allow vegetables to cool at least 30 minutes.
Transfer cooled broccoli and cauliflower to a large mixing bowl.
Add slivered almonds, dried cranberries and celery.
Pour dressing over salad and mix using tongues.
Best if served the same day.