Place the raspberries and rhubarb in a jar. Add the sugar and mash in with the berries using a muddler or wooden spoon. Place in the fridge and let sit for 2 days, shaking occasionally to help dissolve sugars. After two days, pour the mixture into a fine mesh sieve, pressing on the solids to strain out the fruit syrup (alternatively, you can blend the whole mixture and then strain it through a sieve to get the most fruit and flavor out of it). Place the syrup back in a jar and mix with the honey, rose oil(or water) and vinegar. Place back in the fridge for 5-7 days. Shake syrup before using.
SHORT METHOD:
Place the rhubarb and sugar in a saucepan, along with ½ cup of water. Bring to a simmer, stirring to dissolve sugar. Once the sugar is dissolved, remove from heat and let cool completely. Pour mixture into a blender, along with the raspberries, honey, rose oil(or water) and vinegar. Blend until smooth. Run the mixture through a fine mesh sieve and discard solids. Place the syrup in a container and refrigerate for 5-7 days (or it can be used immediately).
Mix 1 part syrup to 4 parts sparkling water and pour over ice. You may like more or less syrup, depending on your preferences.
Recipe by The Kitchen McCabe at https://www.thekitchenmccabe.com/2016/05/18/raspberry-rhubarb-rose-shrubs/