In a bowl, add rhubarb, berries, honey, cinnamon, ginger and vanilla. Combine and pour into a 9×13 in. (3 L) rectangular dish.
For the topping, combine flour and oatmeal in a bowl. Using your fingertips, mix butter and honey into the flour and oatmeal until crumbly, or pulse in a food processor until pebbly.
Add walnuts and stir, then sprinkle over the fruit. Bake, uncovered, until fruit is soft and topping is golden, about half an hour.
You can also cook the crisp on a gas barbecue: Cover crisp with tin foil to make sure it doesn’t burn. Leave the flame on one side set at 325°F and place crisp on the side that has the heat off for approximately half an hour, or until the fruit starts to bubble.