Pour almond milk into blender; add dates and let stand for 10 minutes to soften. Add walnuts and flaxseeds or chia seeds; process on high until nuts and seeds are fully incorporated. Add yogurt, bananas, and frozen raspberries, and process until smooth and creamy. Sweeten to taste with a few drops of stevia, if desired. Pour into glasses and garnish with fresh raspberries and walnuts, if desired. Serve immediately.