Place the almonds in a dry skillet/frying pan and warm over a medium heat, tossing gently now and then so that they become gently brown all over. Remove from the heat and set aside.
As the almonds cook, prepare the rest of the salad. Place the spinach or other greens at the bottom or two plates or bowls (or one larger to serve from).
Score the grapefruit skin in quarters then peel off the outer skin. Using a sharp, thin pairing knife, cut back the outer membrane layer so you can see the fruit's outer edge inside. Carefully cut down the side of the inner membrane on either side so that you can release the grapefruit segments. Either leave whole, or cut in half, and arrange over the spinach. As far as you can, collect the juice as you work to use in the vinaigrette.
Cut the avocado into chunks or slices and arrange over the spinach. Add the toasted almonds.
Squeeze the grapefruit scraps to extract the juice. If needed, strain the juice to remove seeds and pieces of pith/membrane. If you don't quite have enough grapefruit juice (though you likely will), top up with a little lemon juice.
Combine the grapefruit juice, olive oil, honey, salt and pepper in a small bowl and whisk together to combine. Drizzle the vinaigrette over the salad(s) and serve.