Heat vegetable broth in a separate saucepan and maintain at a gentle simmer (around 180°F).
Heat olive oil in a large skillet over medium heat until shimmering.
Add diced onion and cook for 4 minutes until translucent.
Add minced garlic and cook for 1 minute until fragrant.
Add Arborio rice and toast for 2 minutes, stirring constantly until grains turn slightly translucent at the edges.
Pour in 1/2 cup of warm broth and stir continuously until fully absorbed. Continue adding broth 1/2 cup at a time, stirring constantly and waiting for each addition to be absorbed before adding the next (this takes about 18-20 minutes total).
Add asparagus pieces and fresh peas when the rice is halfway cooked (after about 10 minutes of adding broth).
Stir in chopped spring onions during the last 3 minutes of cooking.
Remove skillet from heat when rice is al dente and mixture is creamy.
Stir in grated Parmesan cheese until melted and fully incorporated.
Fold in unsalted butter until the risotto becomes glossy.
Add lemon juice and chopped fresh parsley, stirring to combine evenly.
Let rest for 2 minutes before serving to allow flavors to meld.