G: TLDR: Add onion, celery, carrot, garlic, herbs, and peppercorns — simmer about 45 min to make a good vegetable stock base.

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Core Stock Add-Ins

Since you already have asparagus stalks and leek greens, add:

• 1 large onion (quartered; skin on for color)
• 2–3 celery ribs (with leaves if you have them)
• 1–2 carrots (rough chop)
• 4–6 garlic cloves (crushed)
• 1–2 bay leaves
• ½ tsp black peppercorns
• A few sprigs of parsley or thyme (or both)
• Optional: a few dried mushrooms for umami

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Quick Method

1. Rough-chop everything.
2. Sauté onion, carrot, and celery in a little oil until lightly browned.
3. Add your asparagus and leeks plus about 8 cups (2 L) water.
4. Bring to a boil, then simmer uncovered ~45 min.
5. Strain; discard solids.

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Notes

• Avoid starchy vegetables (potatoes, pumpkin) and cruciferous ones (broccoli, cabbage) — they muddy the flavor.
• If you want a green-flavored stock, add spinach stems or parsley stems near the end and simmer only 10 min more.