Combine the rhubarb, strawberries, sugar, cornstarch, lemon juice, lemon zest and vanilla together in a bowl. Make sure the cornstarch is well combined.
Pour the strawberry and rhubarb mixture into a 2-quart baking (8x8 square) dish.
To make the crisp topping, combine the butter and flour with a fork. Then add the sugar, oats, pecans, cinnamon, and salt and mix until crumbly.
Spoon the topping evenly over the fruit.
Cover lightly with tinfoil and bake for 40 minutes. Then, remove the tinfoil and continue baking for another 20 minutes.
Remove and let it cool for at least 20 minutes or longer. The strawberry rhubarb mixture will thicken as it cools.
Notes
Do not increase the amount of strawberries, as they will add too much liquid.
Best served with ice cream or whipped cream.
It may be stored covered on the counter for up to 3 days.
Store in an airtight container in the refrigerator for up to 5 days, but note that the crisp won’t stay very crisp in the refrigerator.